Q: Which statement best describes the mole in chemistry?

A mole is a fixed number of particles, specifically $6.02 \times 10^{23}$ particles.

Q: A sample contains 75% chlorine-35 and 25% chlorine-37. What is the relative atomic mass ($A_r$) of chlorine?

$A_r = (0.75 \times 35) + (0.25 \times 37) = 35.5$

Q: What is the molar mass ($M$) of sodium hydroxide (NaOH)?

$M = 22.99 + 15.999 + 1.008 = 39.997 \text{ g/mol}$

Q: How many molecules are present in 2 moles of water ($H_2O$)?

$2 \times 6.02 \times 10^{23} = 1.204 \times 10^{24}$ molecules

Q: A 150.0 g sample of copper ore contains 87.3 g of pure copper. What is the percentage purity?

$\frac{87.3}{150.0} \times 100 = 58.2\%$

Q: What is the volume occupied by 3 moles of helium gas at STP?

$3 \times 22.7 = 68.1 \text{ dm}^3$

Question 1

Which statement best describes the mole in chemistry?

Accepted Answer

A mole is a fixed number of particles, specifically $6.02 	imes 10^{23}$ particles.

Question 2

A sample contains 75% chlorine-35 and 25% chlorine-37. What is the relative atomic mass ($A_r$) of chlorine?

Accepted Answer

$A_r = (0.75 	imes 35) + (0.25 	imes 37) = 35.5$

Question 3

What is the molar mass ($M$) of sodium hydroxide (NaOH)?

Accepted Answer

$M = 22.99 + 15.999 + 1.008 = 39.997 	ext{ g/mol}$

Question 4

Which of the following is the correct balanced chemical equation for the reaction of hydrogen and oxygen to form water?

Accepted Answer

$2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$

Question 5

How many molecules are present in 2 moles of water ($H_2O$)?

Accepted Answer

$2 	imes 6.02 	imes 10^{23} = 1.204 	imes 10^{24}$ molecules

Question 6

A solution contains 5.0 g of NaCl dissolved in 250 mL of water. What is the concentration in $g/dm^3$?

Accepted Answer

$\frac{5.0}{0.25} = 20 \ g/dm^3$

Question 7

What is the empirical formula of a compound containing 80% carbon and 20% hydrogen by mass?

Accepted Answer

CH$_3$

Question 8

Which statement about balancing chemical equations is correct?

Accepted Answer

You can only change the coefficients, not the subscripts, to balance an equation.

Question 9

Calculate the number of moles in 80 g of NaOH ($M = 40 	ext{ g/mol}$).

Accepted Answer

$\frac{80}{40} = 2$ moles

Question 10

A 150.0 g sample of copper ore contains 87.3 g of pure copper. What is the percentage purity?

Accepted Answer

$\frac{87.3}{150.0} 	imes 100 = 58.2\%$

Question 11

What is the volume occupied by 3 moles of helium gas at STP?

Accepted Answer

$3 	imes 22.7 = 68.1 	ext{ dm}^3$

Question 12

Which of the following best describes a redox reaction?

Accepted Answer

A reaction involving simultaneous oxidation and reduction.

Question 13

What is the mass of 0.5 moles of bromine molecules ($Br_2$, $M = 159.8 	ext{ g/mol}$)?

Accepted Answer

$0.5 	imes 159.8 = 79.9 	ext{ g}$

Question 14

Which of the following is the correct formula for calculating molar concentration ($c$) in $mol/dm^3$?

Accepted Answer

$c = \frac{	ext{mole of solute}}{	ext{volume of solution (dm}^3)}$

Question 15

A solution contains 0.132 g of LiF in 100.0 g of starch tablet. What is the percentage purity by mass of LiF?

Accepted Answer

$\frac{0.132}{100.0} 	imes 100 = 0.132\%$

Question 16

Which of the following is the correct definition of relative molecular mass ($M_r$)?

Accepted Answer

The sum of the relative atomic masses ($A_r$) of all atoms in a molecule.

Question 17

If you have 0.5 mole of sodium and 0.5 mole of helium, what do they have in common?

Accepted Answer

They both contain $0.5 	imes 6.02 	imes 10^{23}$ particles.

Question 18

Which of the following is NOT a correct use of dimensional analysis in chemistry?

Accepted Answer

Changing the chemical identity of a substance.

Unit 2: Mole-cular Cookbook – Quantitative Chemistry and Chemical Calculations

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