The area from which a river and its tributaries collect water is called the:
a.
Rivertown
c.
Water cycle
b.
Drainage basin
d.
Pull zone

Answer:

Nuclear Chain Reactions. A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons).

Explanation:

A nuclear chain reaction occurs when an initial reaction causes more reactions, generating recurring secondary reactions, forming a self-sustaining chain, much like a domino effect. This process is common in nuclear fission and fusion.

In a nuclear chain reaction, an initial reaction causes excess neutrons to be emitted. These neutrons then cause other similar nuclear reactions, generating more neutrons and thus, more nuclear reactions. This sequence forms a self-sustaining chain. This is common in both nuclear fission, as seen in nuclear power plants and atomic bombs, or in a controlled manner in nuclear fusion, as is being researched for future clean energy solutions. It's like a domino effect but with atomic nuclei and neutrons.

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Answer:

what are the options because a graduated cylinder is a science mixing cylinder is a common piece of laboratory equipment used to measure the volume of a liquid

Explanation:

Explanation:

Metals are the substances or elements that contain more number of electrons.

Some of the properties of metals are as follows.

Answer: The molar mass of magnesium nitrate is 148.3148 g/mol.

Explanation:

The molar mass of [tex]Mg(NO_3)_2[/tex] is approximately 148.33 grams/mole.

Calculating the total mass of one mole of the chemical [tex]Mg(NO_3)_2[/tex] (magnesium nitrate) is necessary to determine its molar mass. The molar mass is determined by adding the atomic masses of all the components of the compound, taking into account the value of each subscript.

The elements involved have the following atomic masses:

Magnesium (Mg) = 24.31 grams/mole

Nitrogen (N) = 14.01 grams/mole

Oxygen (O) = 16.00 grams/mole

Magnesium nitrate [tex](Mg(NO_3)_2)[/tex] consists of one magnesium atom (Mg), two nitrate ions [tex](NO^3^-)[/tex], and their respective subscripts.

This formula can be used to obtain the molar mass:

Molar mass of [tex]Mg(NO_3)_2[/tex]=  (1 × Mg) + (2 × (N + (3 × O)))

Molar mass of [tex]Mg(NO_3)_2[/tex]= (1 × 24.31) + (2 × (14.01 + (3 × 16.00)))

Molar mass of [tex]Mg(NO_3)_2[/tex]= 24.31 + (2 × (14.01 + 48.00))

Molar mass of [tex]Mg(NO_3)_2[/tex]=24.31 + (2 × 62.01)

Molar mass of [tex]Mg(NO_3)_2[/tex]=24.31 + 124.02

Molar mass of [tex]Mg(NO_3)_2[/tex]= 148.33 grams/mole

Therefore, the molar mass of [tex]Mg(NO_3)_2[/tex] is approximately 148.33 grams/mole.

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The salt formed from the reaction between HBr and NaOH is sodium bromide (NaBr), which is the result of a neutralization reaction involving the combination of hydroxide ions from the base with hydrogen ions from the acid, and the combination of sodium ions from the base with bromide ions from the acid.

The salt formed from the acid/base reaction between HBr (hydrobromic acid) and NaOH (sodium hydroxide) is called sodium bromide (NaBr). When these two reactants combine in a neutralization reaction, the hydroxide ions (OH⁻) from the base react with the hydrogen ions (H⁺) from the acid to produce water (H₂O), and the remaining sodium ions (Na⁺) from the base combine with the bromide ions (Br⁻) from the acid to form the salt sodium bromide (NaBr).

This can be represented by the chemical equation:

NaOH (aq) + HBr (aq) rightarrow NaBr (aq) + H2O (l)

A salt is generally an ionic compound made up of the cation from a base and the anion from an acid. As HBr is a strong acid and NaOH is a strong base, the resulting sodium bromide solution has a neutral pH, similar to sodium chloride (NaCl).

Answer:

141.1g

Explanation:

The formula of ammonia is written as NH₃

Given parameters:

Number of particles = 5.0 x 10²⁴atoms

To calculate the mass of the substances that makes up this number of ammonia particles, we follow use the mole concept to derive it.

1. We know that a mole is the amount of a substance that contains the Avogadro's number of particles i.e 6.02 x 10²³

From this, we can then derive the number of moles using the expression below:

Number of moles =

number of particles/6.02 x 10²³

2. With the number of moles, we can deterime the mass using the expression below:

Number of moles= mass/molar mass

Mass(g) = number of moles x molar mass

Solution

From step 1:

Number of moles = 5.0 x 10²⁴ / 6.02 x 10²³

No of moles of NH₃ = 0.83 x 10 = 8.3mol

Step 2:

Molar mass of NH₃:

Atomic mass of N = 14gmol⁻¹

H = 1gmol⁻¹

Molar mass = 14 + (1x3) = 17gmol⁻¹

Using the expression:

Mass(g) = number of moles x molar mass

Mass of NH₃ = 8.3 x 17 = 141.1g

Answer:

The sweetened tea mixture is called homogeneous.

The sweetened tea mixture is a homogeneous mixture unlike the heterogeneous mixture which has 2 phases.

Heterogeneous mixtures is defined as a type of mixture where  the composition is not uniform throughout the mixture.It consists as two or more phases. The phases are chemically distinct  and separate from each other.

As there are two or more phases present in heterogeneous mixtures they can be separated by solvent extraction  method where in one phase is miscible with the solvent while the other phase is immiscible.

Components of a heterogeneous mixtures are distinctly visible . There are two types of heterogeneous mixtures, the colloids and the suspensions.These two differ with each other with respect to particle size.It consists of 2 or more phases.

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Explanation:

Atomic number of sodium is 11 and its electronic distribution is 2, 8, 1. So, it means there is only one valence electron present in a sodium atom. This valence electron is represented by a dot in the Lewis structure.

Atomic number of chlorine is 17 and its electronic distribution is 2, 8, 7. So, it means there are seven valence electrons present in a chlorine atom. These valence electrons are represented by seven dots in the Lewis structure.

Also, in order to attain stability sodium will lose one valence electron and this valence electron will be gained by chlorine atom.

Therefore, an arrow pointed from dot near Na to Cl shows that an electron is being donated from Na to the Cl atom.

Answer:

CH₃CH(CH₃)CH(C₃H₇)CH₂CH(CH₃)₂:

4-isopropyl-2-methylpentane.

Explanation:

Step One: Draw the structure formula of this compound. Parentheses in the formula indicate substitute groups that are connected to the carbon atom to the left.

For example, the first (CH₃) indicates that the second carbon atom from the left is connected to:

Each carbon atom in this compound is connected to four other atoms. All bonds between carbon atoms are single bonds.

The C₃H₇ in the second pair of parentheses is the condensed form of CH₃CH₂CH₂-. See the first sketch attached. Groups in parentheses are highlighted.

Step Two: Find the carbon backbone. The backbone of a hydrocarbon is the longest chain of carbon atoms that runs through the compound. See the second sketch attached. The backbone of this compound consists of seven carbon atoms and is highlighted in green. The name for this backbone shall be heptane.

Step Three: Identify and name the substitute groups.

The two substitute groups are circled in blue in the second sketch.

Step Four: Number the atoms.

Isopropyl shall be placed before methyl. Start from the right end to minimize the index number on all substitute groups. The methyl group is on carbon number two and the isopropyl group on carbon number four. Hence the name:

4-isopropyl-2-methylheptane.

To find the number of molecules in 3.6 grams of NaCl, convert the grams to moles of NaCl using its molar mass and then multiply the number of moles by Avogadro's number (6.02 x 10^23). The result is approximately 3.7 x 10^22 molecules.

To find out how many molecules are in 3.6 grams of NaCl, first calculate the number of moles of NaCl. Each mole of NaCl has a mass of 58.44 grams (sum of atomic masses of Na and Cl). So, 3.6 grams corresponds to 3.6/58.44 = 0.0616 moles.

Now, use the Avogadro's number which states that one mole of any substance contains 6.02 x 10^23 molecules. So, 0.0616 moles of NaCl would contain approximately 0.0616 x 6.02 x 10^23 = 3.7 x 10^22 molecules.

In conclusion, the number of molecules in 3.6 grams of NaCl is approximately 3.7 x 10^22.

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To find the number of molecules in 3.6 grams of NaCl, first calculate the number of moles (0.0615 moles), then convert this to molecules using Avogadro's number, resulting in approximately 3.70 x 10²² molecules.

To determine the number of molecules in 3.6 grams of NaCl, follow these steps:

              Number of moles of NaCl = 3.6 g / 58.5 g/mol ≈ 0.0615 moles

         Number of molecules = 0.0615 moles x 6.022 x 10²² molecules/mole ≈ 3.70 x 10²² molecules

Therefore, there are approximately 3.70 x 10²² molecules in 3.6 grams of NaCl.

Answer:

= 82%

Explanation:

Percentage purity is calculated by the formula;

% purity = (mass of pure chemical/total mass of sample) × 100

In this case;

1 mole of Ca(NO3)2 = 164 g

but; 164 g of Ca(NO3)2 = 40 g Ca

Therefore; mass of Ca(NO3)2 = 164 /40

                                                  = 4.1 g

Thus;

% purity of Ca(NO3)2 = (Mass of Ca(NO3)2/ mass of the sample)× 100

                                    = (4.1 g/ 5 g) × 100

                                    = 82%

To find the mass percent purity of calcium nitrate in the sample, calculate the equivalent mass of calcium nitrate from the given mass of calcium and then use this to determine the purity percentage, which comes out to be 81.82%.

The question asks for the mass percent purity of calcium nitrate in a sample that has been contaminated with silica. It is given that the sample has a total mass of 5.0 g and contains 1.0 g of calcium. Calcium nitrate (Ca(NO₃)₂) has a known molar mass of 164 g/mol. As calcium has a molar mass of 40.08 g/mol, and there is one mole of calcium in one mole of calcium nitrate, the mass of pure calcium nitrate can be calculated by converting the mass of calcium found in the sample to its equivalent mass in calcium nitrate.

First, find the number of moles of calcium in the sample:

moles of Ca = 1.0 g / 40.08 g/mol = 0.024945 moles

The moles of calcium nitrate are equal to the moles of calcium, as there is one calcium atom per molecule of calcium nitrate. Therefore, to find the mass of calcium nitrate:

mass of Ca(NO₃)₂ = 0.024945 moles × 164 g/mol = 4.091 g

The mass percent purity of calcium nitrate can then be calculated:

mass percent purity of Ca(NO₃)₂ = (mass of Ca(NO₃)₂ / total mass of sample) × 100 = (4.091 g / 5.0 g) × 100 = 81.82%

Answer:

- 618.8 J.

Explanation:

Q = m.c.ΔT,

where, Q is the amount of heat released by steam (Q = ??? J).

m is the mass of the steam (m = 15.5 g).

c is the specific heat of the steam (c of steam = 1.996 J/g.°C).

ΔT is the difference between the initial and final temperature (ΔT = final T - initial T = 100.0°C - 120.0°C = - 20.0°C).

∴ Q = m.c.ΔT = (15.5 g)(1.996 J/g.°C)(- 20.0°C) = - 618.8 J.

Answer:NF3 is the correct formula for nitrogen trifluoride.

Explanation:

In the given question, the correct formula for nitrogen trifluoride is [tex]\rm NF_3[/tex].

Nitrogen trifluoride is a covalent compound that is composed of one nitrogen atom and three fluorine atoms.

The nitrogen atom shares one of its valence electrons with each of the three fluorine atoms to form three single covalent bonds. The resulting molecule has a trigonal pyramidal shape, with the nitrogen atom at the center and the three fluorine atoms arranged around it.

The prefix "tri-" in the name of the compound indicates that there are three fluorine atoms in the molecule.

Therefore, [tex]\rm NF_3[/tex] is the correct formula for nitrogen trifluoride.

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Answer:

It would be 30km

Explanation:

The approximate straight-line distance between Red Bank and Belleville, presumably in New Jersey, USA, is about 30 kilometers, making 'option D: 30 km' the correct choice from the given options.

The distance between Red Bank and Belleville can vary based on the specific locations in each place, the method of travel, and the route that is chosen. However, to determine the approximate distance between these two locations, one might use a mapping service or tool. Assuming these are the towns in New Jersey, USA, and using a direct path (as the crow flies), the distance is about 30 kilometers. Therefore, out of the options provided, the answer would be option D: 30 km. Note that real-world road distances might be different because roads do not always make straight, direct paths between locations.

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Answer:

the answer is elevation

Answer: C. Elevation

Answer:

The reaction will move to the left.

Explanation:

Ba(OH)₂ = Ba²⁺ + 2OH⁻,

Ba(OH)₂ is dissociated to Ba²⁺ and 2OH⁻.

H⁺ will combine with OH⁻ to form water.

So, the concentration of OH⁻ will decrease and the equilibrium is disturbed.

According to Le Châtelier's principle: when there is an dynamic equilibrium, and this equilibrium is disturbed by an external factor, the equilibrium will be shifted in the direction that can cancel the effect of the external factor to reattain the equilibrium.

The base will dissociate to form more OH⁻ and thus, the quantity of Ba(OH)₂ will decrease.

So, the right choice is: the reaction will move to the left, is the choice that will not happen to the equilibrium.

Final answer:

Adding H+ ions to a solution of Ba(OH)₂ shifts the equilibrium to the left, reducing the dissociation of Ba(OH)₂ and forming water by combining H₊ and OH₋ ions. The increase in H₊ leads to an increase in Ba(OH)₂ concentration as OH₋ ions are consumed.

Explanation:

When an acid is added to a solution of Ba(OH)₂, which is barium hydroxide, equilibrium dynamics according to Le Chatelier's principle tell us that the H₊ ions will react with OH₋ ions to form water. This reaction will cause the equilibrium of the solution to shift. Specifically, the ionization of Ba(OH)₂ which is represented by the reaction Ba(OH)₂ ⇒ Ba₂₊ + 2OH₋, will move towards the left to decrease the concentration of OH₋ ions that combine with incoming H₊ ions.

The reaction that will not happen is the dissociation of more Ba(OH)₂ to form additional OH₋ ions, because of the presence of excess H₊ ions that drive the equilibrium leftwards. The quantity of Ba(OH)₂ will increase at the expense of OH₋ ions, as they are consumed to form water.

Answer:

A negatron emission

Explanation:

We know that radioactivity orginates from instability of the nucleus. When the nucleus is unstable, radioactive emissions are produced in the form of any of these rays:

> Alpha particle emisson

>Beta particles

> Gamma rays

These emissions create a balance for a radioactive decay.

In balancing nuclear reactions we make sure that the charges on both sides must be conserved and that the mass number and atomic numbers conserved too. This means that the sum of mass number and atomic numbers on both side of the reaction must be equal.

The nucleons are the protons and neutrons, they add up to give the mass number. The atomic number is the proton number.

For the given radioactive reaction:

²³¹₉₀Th → ²³¹₉₁Pa + ?

From this equation, we see that the mass number is conserved but the atomic number is not.

The mass number is the superscript whereas the atomic number is the subscript.

Let's say the decay produces an emission of a particle denoted by X

²³¹₉₀Th → ²³¹₉₁Pa + ᵃₙX

What would the nature of X be?

For the charges and masses to be conserved, X must have mass number of 0 and an atomic number of -1.

Checking:

Mass number:

231 = 231 + a ( a is the mass number)

a = 231 - 231 = 0

Atomic number:

90 = 91 + n

n = 90- 91 = - 1

With X having a mass number of 0 and an atomic number of -1, we have a beta particle emission. Specifically, a negatron has been emitted.

A negatron is denoted as ⁰₋₁β which perfectly makes the equation conserved and suits the description of X.

The complete equation is thus written as:

²³¹₉₀Th → ²³¹₉₁Pa + ⁰₋₁β + energy

In the decay of thorium-231 to protactinium-231, a neutron inside the thorium nucleus converts to a proton, resulting in the emission of a beta particle to conserve charge and nucleon number.

The decay of the element thorium-231 (231/90Th) to protactinium-231 (231/91Pa) involves the emission of a decay particle. According to the laws of conservation of charge and nucleon number, we can determine the type of decay and the particle emitted by examining the change in atomic and mass numbers.

231/90Th decays to 231/91Pa which indicates that the mass number has not changed (indicating the total number of protons and neutrons remains constant) but the atomic number has increased by one. This suggests that a neutron has transformed into a proton and the particle emitted must account for the negative charge to conserve the total charge. Since a neutron (n) converting to a proton (p) emits a beta-minus particle ({{e}}) (an electron), which has a negative charge, the decay particle is therefore identified as a beta particle.

Answer:

1. Theory

2. evidence

3. theory

4. Today, natural sciences are more commonly divided into life sciences, such as botany and zoology; and physical sciences, which include physics, chemistry, geology, astronomy, and materials science.

5.In everyday language a theory means a hunch or speculation. Not so in science. In science, the word theory refers to a comprehensive explanation of an important feature of nature supported by facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena".

7.Empirical evidence is information acquired by observation or experimentation. Scientists record and analyze this data. The process is a central part of the scientific method

8. Field research is a research conducted in the real world or a natural setting. It tends to observe, analyze, and describe what exists rather than manipulating a factor under study .On the other hand, controlled laboratory research is a research conducted in a setting specifically designed for research

10.You would say Scientific knowledge is always being changed and performed in different ways. Scientific knowledge is always being changed as scientist are experiencing and collecting new information everyday using the scientific method.

6.In general, a scientific law is the description of an observed phenomenon. It doesn't explain why the phenomenon exists or what causes it. The explanation of a phenomenon is called a scientific theory. It is a misconception that theories turn into laws with enough research.

Answer:

1. The limiting reactant is H₂O.

2. The theoretical yield of Al₂O₃ = 978.8 g.

Explanation:

1. Which is the limiting reactant?

2Al + 3H₂O → Al₂O₃ + 3H₂,

It is clear that 2.0 moles of Al react with 3.0 moles of H₂O to produce 1.0 moles of Al₂O₃ and 3.0 mole of H₂.

∴ 4.8 mol of Al (the remaining 1.7 mol is in excess) reacts completely with 7.2 mol of H₂O with (2: 3) molar ratio.

the limiting reactant is H₂O and the excess reactant is Al.

2. Calculate the theoretical yield.

Using cross multiplication:

2.0 moles of Al produce → 1.0 moles of Al₂O₃.

4.8 moles of Al produce → ??? moles of Al₂O₃.

∴ The no. of moles of Al₂O₃ produced = (1.0 mol)(4.8 mol)/(2.0 mol) = 9.6 mol.

∴ The theoretical yield of Al₂O₃ = (no. of moles)(molar mass) = (9.6 mol)(101.96 g/mol) = 978.8 g.

Answer:

[tex]\boxed{\text{pH = 5.00}}[/tex]

Explanation:

Step 1. Calculate the pOH

pOH =-log[OH⁻]

pOH =-log(1.0 × 10⁻⁹)

pOH = 9.00

Step 2. Calculate the pH

pH + pOH = 14.00

pH + 9.00 = 14.00

[tex]\boxed{\textbf{pH = 5.00}}[/tex]

Final answer:

The pH of a solution with a hydroxide ion concentration of 1.0 × 10⁻⁹ M is 5, indicating the solution is acidic.

Explanation:

To calculate the pH of a solution with a known hydroxide ion concentration, we can use the relationship between the concentration of hydroxide ions ([OH-]) and hydrogen ions ([H+]), which is given by the ionic product of water (Kw = 1.0 × 10⁻¹⁴ at 25°C). The pH is defined as the negative logarithm of the hydrogen ion concentration, pH = -log([H+]).

Since we have the hydroxide ion concentration ([OH-] = 1.0 × 10⁻⁹ M) we can find the hydrogen ion concentration from Kw:

Kw = [H+] × [OH-]

[H+] = Kw / [OH-]

[H+] = (1.0 × 10⁻¹⁴ M2) / (1.0 × 10⁻⁹ M)

[H+] = 1.0 × 10-5 M

Now we can calculate the pH:

pH = -log([H+])

pH = -log(1.0 × 10⁻⁵)

pH = 5

A pH of 5 indicates that the solution is acidic, as a pH less than 7 is characteristic of an acidic solution.

Answer: It would be paint.

Explanation: it would be paint because it doesn't come completely together. another example of a heterogeneous mixture could be cereal and milk.

A decrease in temperature [E] is generally not a cause of chemical reactions. Other factors like the transfer of electrons, formation of a solid, water or gas are common causes.

The most common factors that can cause chemical reactions to occur include the transfer of electrons, formation of a solid, formation of water, and formation of a gas. However, the option that does not generally cause a chemical reaction to occur is a decrease in temperature [E]. In fact, an increase in temperature typically facilitates chemical reactions as it increases the energy of the particles involved, making them more likely to react.

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Most commonly, chemical reactions occur due to the transfer of electrons, the formation of a solid, the formation of water, or the formation of a gas. A decrease in temperature is not typically a factor that facilitates chemical reactions.

The most common factors that cause chemical reactions to occur include the transfer of electrons, the formation of a solid, the formation of water, and the formation of a gas. These actions involve breaking and forming chemical bonds that result in new substances. However, a decrease in temperature is typically not a factor that causes chemical reactions. In fact, an increase in temperature is more likely to cause a chemical reaction as it provides the necessary energy for the particles to react.

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Answer:

Water cycle

Explanation:

The water from the ocean and sea evaporates into the atmosphere and goes into the sky to form clouds.

This turns the liquid water into water vapor in the atmosphere. Plants, too, help water get into the atmosphere through a process called transpiration! After absorbing water from the ground, plants “sweat” water vapor through their leaves to stay cool. Water can also get into the atmosphere from snow and ice.

Answer: A. one atom

Explanation: i did some research on it and that is what i found.

One molecule of H₂SO₄ comprises one atom of sulfur. Thus, option A is correct.

Acid rain is given as the precipitation that has been comprising the chemical compound acidic in nature, thereby the pH of the precipitation as well.

The number of atoms in a molecule can be determined with the chemical formula. The subscript to an element in the formula is corresponding to the number of atoms of element present in the molecule or atom.

In the molecule of H₂SO₄,  the subscript number to the hydrogen atom is 2, thereby 2 hydrogens are present, and the subscript to the sulfur is the representation of the number of atoms of sulfur in the molecule that stands for 1.  

Therefore, 1 atom of sulfur is present in the molecule of H₂SO₄. Thus, option A is correct.

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The Geiger Counter. Geiger counters are used to detect radioactive emissions, most commonly beta particles and gamma rays. The counter consists of a tube filled with an inert gas that becomes conductive of electricity when it is impacted by a high-energy particle.

Hope That Helps!!!

NOTE:Mark as BRAINLIEST!!!!!

Answer:

Geiger counters can detect all kinds of radiation, while scintillation counters can only detect ionizin radiation

Answer:

The train is moving at the speed of 22.36 meters per second

Explanation:

Speed of the train  = 50 .0 miles/hour

1 mile = 1.61 km

50 miles = [tex]50.0\times 1.61 km=80.5 km[/tex]

1 km = 1000 m

1 hour = 3600 seconds

So,the speed of the train in meter per seconds:

[tex]=\frac{80.5\times 1000 m}{3600 s}=22.36 m/s[/tex]

Answer:

Homogeneous because if you were to take a sip it would be the same all throughout the juice, nothing changes. Hope this helped, please mark me the brainliest answer? Have a good day :)

Answer:

Explanation:

heterogeneous

Answer:

1st answer B. Bromine

2nd answer B. magnesium

Explanation:

Hello There!

Which of the following is a non metal that remains liquid at room temperature? "OPTION B"

Chlorophyll is a naturally occurring chelate compound in which central metal is "OPTION B"

The answer is:

The density is equal to:

[tex]78.6\frac{lb}{ft^{3} }[/tex]

This a unit conversion problem, so, converting  the density  of glycerin from [tex]gram/cm^{3}[/tex] to [tex]\frac{lb}{ft^{3}}[/tex]  using the given conversion rates, we have:

[tex]1.26\frac{gram}{cm^{3}}*\frac{1lb}{454gram}*\frac{28317cm^{3} }{1ft^{3} }=78.59\frac{lb}{ft^{3} }[/tex]

Rounding to three significant figures, we have that the density is equal to:

[tex]78.6\frac{lb}{ft^{3} }[/tex]

Have a nice day!

The density is 78.6

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The introduction of the measles vaccine in the US in 1960 led to a significant decrease in the number of measles cases.

The information on the graph shows the number of measles cases in the US between 1954 and 2008. The graph shows that the measles vaccine was introduced in 1960. From the graph, we can conclude that the introduction of the measles vaccine led to a significant decrease in the number of measles cases in the US. Prior to the vaccine, there were fluctuating numbers of cases, but after the vaccine was introduced, the number of cases dropped to almost zero.