Q: A sample contains $$5.00$$ moles of sodium ($$\mathrm{Na}). $$What is the total mass of sodium in the sample? (Atomic mass of $$\mathrm{Na} = 22.99\ \mathrm{g/mol})$$

$$5.00\ \mathrm{mol} \times 22.99\ \mathrm{g/mol} = 114.95\ \mathrm{g}$$

Q: How many potassium ($$\mathrm{K}) $$atoms are present in $$5.00\ \mathrm{g} of $$potassium? (Atomic mass of $$\mathrm{K} = 39.10\ \mathrm{g/mol})$$

$$5.00\ \mathrm{g} \div 39.10\ \mathrm{g/mol} \times 6.022 \times 10$$^{23}$$ = 7.70 \times 10$$^{22}$$ atoms

Q: Calculate the percent by mass of oxygen in water ($$\mathrm{H_2O}). ($$Atomic masses: $$\mathrm{H} = 1.008\ \mathrm{g/mol}$$, $$\mathrm{O} = 16.00\ \mathrm{g/mol})$$

$$\frac{16.00}{2 \times 1.008 + 16.00} \times 100\% = 88.8\%$$

Q: A compound has the formula $$\mathrm{C_2H_6O}. $$What is its molar mass?

$$2 \times 12.01 + 6 \times 1.008 + 16.00 = 46.07\ \mathrm{g/mol}$$

Q: Which of the following statements about molecular and ionic compounds is correct?

One mole of a molecular compound contains $$6.022 \times 10$$^{23}$$ molecules, while one mole of an ionic compound contains 6.022$$ \times $$10^{23}$$ formula units.

Q: A student is given $$200\ \mathrm{g} of$$ $$\mathrm{C_2H_6O}. $$How many grams of carbon are present in the sample? (Atomic mass: $$\mathrm{C} = 12.01\ \mathrm{g/mol}$$; molar mass of $$\mathrm{C_2H_6O} = 46.07\ \mathrm{g/mol})$$

$$200\ \mathrm{g} \times \frac{2 \times 12.01}{46.07} = 104.2\ \mathrm{g}$$

Question 1

Which of the following best describes Avogadro's number and its significance in chemistry?

Accepted Answer

It is the number of particles (atoms, molecules, or formula units) in one mole of a substance, equal to $$6.022 	imes 10$$^{23}$$.

Question 2

A sample contains $$5.00$$ moles of sodium ($$\mathrm{Na}). $$What is the total mass of sodium in the sample? (Atomic mass of $$\mathrm{Na} = 22.99\ \mathrm{g/mol})$$

Accepted Answer

$$5.00\ \mathrm{mol} 	imes 22.99\ \mathrm{g/mol} = 114.95\ \mathrm{g}$$

Question 3

How many potassium ($$\mathrm{K}) $$atoms are present in $$5.00\ \mathrm{g} of $$potassium? (Atomic mass of $$\mathrm{K} = 39.10\ \mathrm{g/mol})$$

Accepted Answer

$$5.00\ \mathrm{g} \div 39.10\ \mathrm{g/mol} 	imes 6.022 	imes 10$$^{23}$$ = 7.70 	imes 10$$^{22}$$ atoms

Question 4

Which of the following correctly defines the formula mass of a compound?

Accepted Answer

The sum of the atomic masses of all atoms in a molecule or formula unit, measured in amu.

Question 5

Calculate the percent by mass of oxygen in water ($$\mathrm{H_2O}). ($$Atomic masses: $$\mathrm{H} = 1.008\ \mathrm{g/mol}$$, $$\mathrm{O} = 16.00\ \mathrm{g/mol})$$

Accepted Answer

$$\frac{16.00}{2 	imes 1.008 + 16.00} 	imes 100\% = 88.8\%$$

Question 6

A compound has the formula $$\mathrm{C_2H_6O}. $$What is its molar mass?

Accepted Answer

$$2 	imes 12.01 + 6 	imes 1.008 + 16.00 = 46.07\ \mathrm{g/mol}$$

Question 7

If you have $$12.0\ \mathrm{g} of $$nickel ($$\mathrm{Ni}$$), how many moles of nickel do you have? (Atomic mass of $$\mathrm{Ni} = 58.69\ \mathrm{g/mol})$$

Accepted Answer

$$12.0\ \mathrm{g} \div 58.69\ \mathrm{g/mol} = 0.205\ \mathrm{mol}$$

Question 8

Which of the following statements about molecular and ionic compounds is correct?

Accepted Answer

One mole of a molecular compound contains $$6.022 	imes 10$$^{23}$$ molecules, while one mole of an ionic compound contains 6.022$$ 	imes $$10^{23}$$ formula units.

Question 9

A student is given $$200\ \mathrm{g} of$$ $$\mathrm{C_2H_6O}. $$How many grams of carbon are present in the sample? (Atomic mass: $$\mathrm{C} = 12.01\ \mathrm{g/mol}$$; molar mass of $$\mathrm{C_2H_6O} = 46.07\ \mathrm{g/mol})$$

Accepted Answer

$$200\ \mathrm{g} 	imes \frac{2 	imes 12.01}{46.07} = 104.2\ \mathrm{g}$$

Question 10

Which calculation would you use to determine the number of moles in $$50.0\ \mathrm{g} of $$water ($$\mathrm{H_2O}$$)? (Molar mass of $$\mathrm{H_2O} = 18.016\ \mathrm{g/mol})$$

Accepted Answer

$$50.0\ \mathrm{g} \div 18.016\ \mathrm{g/mol}$$

Question 11

A chemist needs to prepare $$100.0\ \mathrm{g} of $$sodium chloride ($$\mathrm{NaCl}). $$How many moles of $$\mathrm{NaCl}$$ does this represent? (Molar mass of $$\mathrm{NaCl} = 58.44\ \mathrm{g/mol})$$

Accepted Answer

$$100.0\ \mathrm{g} \div 58.44\ \mathrm{g/mol} = 1.71\ \mathrm{mol}$$

Question 12

Which of the following best explains why the percent composition by mass is useful in chemistry?

Accepted Answer

It allows chemists to determine the relative amounts of each element in a compound, which is essential for empirical formula determination.

Question 13

A $$10.0\ \mathrm{g}$$ sample of a compound contains $$2.00\ \mathrm{g} of $$hydrogen. What is the percent by mass of hydrogen in the compound?

Accepted Answer

$$\frac{2.00}{10.0} 	imes 100\% = 20.0\%$$

Question 14

Which of the following is the correct formula to calculate the mass percent of an element A in a compound?

Accepted Answer

$$\frac{	ext{Number of atoms of A} 	imes 	ext{Atomic mass of A}}{	ext{Molar mass of compound}} 	imes 100\%$$

Question 15

A $$46.07\ \mathrm{g}$$ sample of $$\mathrm{C_2H_6O}$$ contains what percent by mass of hydrogen? (Atomic mass of $$\mathrm{H} = 1.008\ \mathrm{g/mol})$$

Accepted Answer

$$\frac{6 	imes 1.008}{46.07} 	imes 100\% = 13.1\%$$

Question 16

If a sample contains $$1.00\ \mathrm{mol} of$$ $$\mathrm{H_2O}$$, how many molecules of water are present?

Accepted Answer

$$6.022 	imes 10$$^{23}$$ molecules

Question 17

A $$100.0\ \mathrm{g}$$ sample of $$\mathrm{C_2H_6O}$$ contains how many moles of $$\mathrm{C_2H_6O}$$? (Molar mass $$= 46.07\ \mathrm{g/mol})$$

Accepted Answer

$$100.0\ \mathrm{g} \div 46.07\ \mathrm{g/mol} = 2.17\ \mathrm{mol}$$

Question 18

Which of the following best describes the difference between atomic mass and molar mass?

Accepted Answer

Atomic mass is the mass of a single atom in amu, while molar mass is the mass of one mole of atoms in grams.

Chemical Quantities, Moles, and Composition of Compounds 09-03-2026

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