Q: A sample contains $$0.0472$$ mol of carbon. How many carbon atoms are present in the sample?

$$0.0472 \times 6.022 \times 10$$^{23}$$ = 2.84 \times 10$$^{22}$$ atoms

Q: Given $$5.632 g of$$ $$\mathrm{CH_4$}$$, how many hydrogen atoms are present? (Molar mass of $$\mathrm{CH_4$}$$ = $$16.04 g/$$mol)

$$\frac{5.632}{16.04} \times 4 \times 6.022 \times 10$$^{23}$$ = 8.46 \times 10$$^{23}$$ atoms

Q: Which of the following is Avogadro's number, and what does it represent?

$$6.022 \times 10$$^{23}$$, the number of atoms or molecules in one mole of a substance

Q: A reaction produces $$20 g of$$ $$\mathrm{SO_3$}$$ from $$30 g of$$ $$\mathrm{SO_2$}$$ and excess $$\mathrm{O_2$}. $$What is the percent yield if the theoretical yield is $$37.5 g$$?

$$\frac{20}{37.5} \times 100 = 53.3\%$$

Q: In the reaction $$2\mathrm{Na_2O_2} + 2\mathrm{H_2O} \rightarrow 4\mathrm{NaOH} + \mathrm{O_2$}$$, how many grams of $$\mathrm{O_2$}$$ are produced from $$12.33 g of$$ $$\mathrm{Na_2O_2}$$? (Molar mass $$\mathrm{Na_2O_2}$$ = $$78.0 g/$$mol, $$\mathrm{O_2$}$$ = $$32.0 g/$$mol)

$$\frac{12.33}{78.0} \times \frac{1}{2} \times 32.0 = 2.53 g$$

Q: A chemist combusts $$5.00 g of$$ $$\mathrm{C_2H_5OH} ($$ethanol) in excess $$\mathrm{O_2$}. $$What is the mass of $$\mathrm{CO_2$}$$ produced? (Molar mass $$\mathrm{C_2H_5OH}$$ = $$46.07 g/$$mol, $$\mathrm{CO_2$}$$ = $$44.01 g/$$mol)

$$\frac{5.00}{46.07} \times 2 \times 44.01 = 9.55 g$$

Q: What is the percent composition by mass of oxygen in glucose ($$\mathrm{C_6H_{12}O_6$}$$)? (Atomic masses: C = 12, H = 1, O = 16)

$$\frac{6 \times 16}{180} \times 100 = 53.3\%$$

Q: If $$0.8 \times 10$$^{22}$$ atoms of copper are present, how many moles of copper does this represent?

$$\frac{0.8 \times 10$$^{22}$$}{6.022 \times 10$$^{23}$$} = 0.013$$ mol

Question 1

Which of the following best defines the term 'molecular weight' as used in stoichiometry?

Accepted Answer

The sum of the atomic masses of all atoms in a molecule, expressed in atomic mass units (amu).

Question 2

What is the formula weight of sulfuric acid ($$\mathrm{H_2SO_4}$$)? (Atomic masses: H = 1, S = 32, O = 16)

Accepted Answer

$$\mathrm{2(1) + 32 + 4(16) = 98\ amu}$$

Question 3

A sample contains $$0.0472$$ mol of carbon. How many carbon atoms are present in the sample?

Accepted Answer

$$0.0472 	imes 6.022 	imes 10$$^{23}$$ = 2.84 	imes 10$$^{22}$$ atoms

Question 4

How many moles of oxygen are in $$0.312$$ mol of $$\mathrm{Al(NO_3$)_3}$$?

Accepted Answer

$$0.312 	imes 9 = 2.81$$ mol

Question 5

Given $$5.632 g of$$ $$\mathrm{CH_4$}$$, how many hydrogen atoms are present? (Molar mass of $$\mathrm{CH_4$}$$ = $$16.04 g/$$mol)

Accepted Answer

$$\frac{5.632}{16.04} 	imes 4 	imes 6.022 	imes 10$$^{23}$$ = 8.46 	imes 10$$^{23}$$ atoms

Question 6

Which of the following is Avogadro's number, and what does it represent?

Accepted Answer

$$6.022 	imes 10$$^{23}$$, the number of atoms or molecules in one mole of a substance

Question 7

A reaction produces $$20 g of$$ $$\mathrm{SO_3$}$$ from $$30 g of$$ $$\mathrm{SO_2$}$$ and excess $$\mathrm{O_2$}. $$What is the percent yield if the theoretical yield is $$37.5 g$$?

Accepted Answer

$$\frac{20}{37.5} 	imes 100 = 53.3\%$$

Question 8

In the reaction $$2\mathrm{Na_2O_2} + 2\mathrm{H_2O} \rightarrow 4\mathrm{NaOH} + \mathrm{O_2$}$$, how many grams of $$\mathrm{O_2$}$$ are produced from $$12.33 g of$$ $$\mathrm{Na_2O_2}$$? (Molar mass $$\mathrm{Na_2O_2}$$ = $$78.0 g/$$mol, $$\mathrm{O_2$}$$ = $$32.0 g/$$mol)

Accepted Answer

$$\frac{12.33}{78.0} 	imes \frac{1}{2} 	imes 32.0 = 2.53 g$$

Question 9

Which reactant is the limiting reagent when $$1.50$$ mol of $$\mathrm{N_2$}$$ reacts with $$2.00$$ mol of $$\mathrm{H_2$} to $$form $$\mathrm{NH_3$}$$ according to $$\mathrm{N_2$} + 3\mathrm{H_2$} \rightarrow 2\mathrm{NH_3$}$$?

Accepted Answer

$$\mathrm{H_2$}$$

Question 10

A chemist combusts $$5.00 g of$$ $$\mathrm{C_2H_5OH} ($$ethanol) in excess $$\mathrm{O_2$}. $$What is the mass of $$\mathrm{CO_2$}$$ produced? (Molar mass $$\mathrm{C_2H_5OH}$$ = $$46.07 g/$$mol, $$\mathrm{CO_2$}$$ = $$44.01 g/$$mol)

Accepted Answer

$$\frac{5.00}{46.07} 	imes 2 	imes 44.01 = 9.55 g$$

Question 11

What is the percent composition by mass of oxygen in glucose ($$\mathrm{C_6H_{12}O_6$}$$)? (Atomic masses: C = 12, H = 1, O = 16)

Accepted Answer

$$\frac{6 	imes 16}{180} 	imes 100 = 53.3\%$$

Question 12

If $$0.8 	imes 10$$^{22}$$ atoms of copper are present, how many moles of copper does this represent?

Accepted Answer

$$\frac{0.8 	imes 10$$^{22}$$}{6.022 	imes 10$$^{23}$$} = 0.013$$ mol

Question 13

Which of the following is the correct balanced equation for the reaction of calcium chloride with hydrochloric acid?

Accepted Answer

$$2\mathrm{HCl}_{(aq)} \rightarrow \mathrm{Ca}^{2+}_{(aq)} + 2\mathrm{Cl}^-_{(aq)}$$

Question 14

A limiting reactant problem involves $$40.0 g of$$ $$\mathrm{C_8H_{18}}$$ and excess $$\mathrm{O_2$}. $$What is the theoretical yield of $$\mathrm{CO_2$}$$? (Molar mass $$\mathrm{C_8H_{18}}$$ = $$114 g/$$mol, $$\mathrm{CO_2$}$$ = $$44 g/$$mol, $$\mathrm{C_8H_{18}} + 12.5\mathrm{O_2$} \rightarrow 8\mathrm{CO_2$} + 9\mathrm{H_2O})$$

Accepted Answer

$$\frac{40.0}{114} 	imes 8 	imes 44 = 123.5 g$$

Question 15

Which of the following best describes the concept of percent yield in a chemical reaction?

Accepted Answer

The ratio of the actual yield to the theoretical yield, multiplied by 100.

Question 16

In a reaction, $$0.231$$ mol of $$\mathrm{SO_2$} is $$mixed with excess $$\mathrm{O_2$} to $$produce $$\mathrm{SO_3$}. If $$the reaction is $$2\mathrm{SO_2$} + \mathrm{O_2$} \rightarrow 2\mathrm{SO_3$}$$, how many moles of $$\mathrm{SO_3$}$$ are produced?

Accepted Answer

$$0.231$$ mol

Question 17

Which of the following is the correct formula for calculating the percent composition of an element in a compound?

Accepted Answer

$$\frac{	ext{mass of element}}{	ext{formula mass}} 	imes 100$$

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