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High School Chemistry : Units

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Example Question #11 : Using Avogadro's Number

How many molecules are in $5.2mol\:KCl$ ?

Possible Answers:

$3.13\cdot10^{22}\text{molecules}$

$3.13\cdot10^{24}\text{molecules}$

$5.14\cdot10^{24}\text{molecules}$

$6.02\cdot10^{23}\text{molecules}$

None of these

Correct answer:

$3.13\cdot10^{24}\text{molecules}$

Explanation:

According to Avogadro's law, each mole of a compound contains $6.02\cdot10^{23}\text{molecules}$ . Use dimensional analysis to find the number of molecules in 5.2mol .

$5.2mol\: KCl\cdot \frac{6.02\cdot 10^{23}\text{molecules}}{1mol}=3.13\cdot10^{24}\text{molecules}$

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Example Question #11 : Using Avogadro's Number

Determine the number of sodium atoms in a 150g block of sodium chloride.

Possible Answers:

$2.05*10^{24}atoms$

155atoms

$1.55*10^{24}atoms$

1.55atoms

$2.27*10^{24}atoms$

Correct answer:

$1.55*10^{24}atoms$

Explanation:

Use dimensional analysis:

$150gNaCl*\frac{1molNaCl}{58.44gNaCl}*\frac{1 mol Na}{1 mol NaCl}*\frac{6.02*10^{23}atoms}{1 mol}=1.55*10^{24}atoms$

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Example Question #12 : Using Avogadro's Number

A container has $4.33\times 10^{24}$ molecules of gas in it. How many moles of the gas are in the container?

Possible Answers:

4.33 moles

7.19 moles

Not enough information to solve the problem.

0.14 moles

Correct answer:

7.19 moles

Explanation:

Avogadro's number tells us how many molecules of gas are in one mole of the container. We are essentially doing a unit conversion from "number of molecules" to moles -

$4.33\times 10^{24} molecules\left ( \frac{1 mole}{6.022\times 10^{23} molecules} \right )=7.19 moles$

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Example Question #1 : Concentration And Units

What is the concentration of Ca in a solution of 1 mol CaCl2 in 1 L of distilled water? (M = molarity, m= molality)

Possible Answers:

Cannot be determined

1 M

2 M

1 m

2 m

Correct answer: 1 m

Explanation:

The definition of molality is moles of solute in 1 kg of the solvent, whereas molarity is the number of moles of solute per 1 L of solutioin. Since 1 mol of CaCl2 is added to 1 L of water, this means that the volume of the final solution is greater than 1 L. Thus, molality is the more accurate concentration determinant, since the solution is probably close to 1 L.

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Example Question #1 : Concentration And Units

Which of the following is equivalent to molarity?

Possible Answers:

Moles of solute per liter of solution

Moles of solute per kilogram of solvent

Kilograms of solute per mole of solution

Moles of solute per liter of solvent

Correct answer:

Moles of solute per liter of solution

Explanation:

Molarity, molality, and normality are the three principle ways to measure concentration. Molarity is a measure of moles of solute per liter of solution. Molality is a measure of moles of solute per kilogram of solvent. Normality expressly relates to acids and bases, and is the measure of moles of solute divided by the number of hydrogen equivalents per mole, all divided by liters of solution. Normality is also referred to as "equivalents (of acid) per liter."

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Example Question #1 : Help With Concentration Units

How many moles of HCl are in of a 0.1M solution?

Possible Answers:

1.0 mol

2.5 mol

10.0 mol

0.5 mol

5.0 mol

Correct answer:

0.5 mol

Explanation:

Molarity is defined as moles of solute divided into liters of solution. We can set up the equation as follows:

$0.1M = \frac{x}{5 L}$

Set the left side of the equation over and solve as a proportion.

$\frac{0.1M}{1} = \frac{x}{5 L}$

Cross multiply.

1x=0.1M*5L

Remember molarity is defined as moles of solute per liter of solution.

$M=\frac{mol}{L}$

Substiute.

$x=\frac{0.1mol}{L}*\frac{5L}{1}$

Liters cancel out.

x=0.5mol

Solve for the number of moles. This gives us 0.5 mol of solute in a 0.1 M , solution of HCl .

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Example Question #1 : Help With Concentration Units

Which of the following units is common for measuring concentration?

Possible Answers:

$\frac{mol}{L}$

$\frac{kg \cdot m}{s^2}$

mmHg

atm

$\frac {kg}{m^3}$

Correct answer:

$\frac{mol}{L}$

Explanation:

$\frac{mol}{L}$ is the dimensional breakdown for the commonly used unit , or molarity, for concentration.

While the other units might be seen in chemistry, they are used for other topics.

Millimeters of mercury (mmHg) and atmospheres (atm) are seen in regards to pressure.

$\frac {kg}{m^3}$ is the dimensional breakdown for density. And $\frac{kg \cdot m}{s^2}$ is the dimensional breakdown for a Newton, also written as N. This is associated with force.

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High School Chemistry : Units

Study concepts, example questions & explanations for High School Chemistry

All High School Chemistry Resources

Example Questions

Example Question #11 : Using Avogadro's Number

Example Question #11 : Using Avogadro's Number

Example Question #12 : Using Avogadro's Number

Example Question #1 : Concentration And Units

Example Question #1 : Concentration And Units

Example Question #1 : Help With Concentration Units

Example Question #1 : Help With Concentration Units

All High School Chemistry Resources

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