Stoichiometry — Practice Quiz

A Chemistry cheat sheet for Stoichiometry — every key formula with its symbols defined — plus a medium-level practice quiz to test recall.

Formulas & key concepts

Mass Percentage: Calculates the percentage by mass of an element in a compound.

$$\% \text{ element} = \frac{(\text{number of atoms})(\text{atomic weight})}{\text{formula weight of compound}} \times 100\%$$

Percent Yield: The ratio of the actual yield to the theoretical yield expressed as a percent.

$$\% \text{ yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\%$$

Avogadro's Number: The number of constituent particles (usually atoms or molecules) that are contained in the amount of substance given by one mole.

$$N_A = 6.022 \times 10^{23} \text{ mol}^{-1}$$

Practice quiz

  1. What is the mass percentage of oxygen in water ($H_2O$)? (Atomic weight of $H = 1.008 \text{ g/mol}$, $O = 15.999 \text{ g/mol}$)

    • A) $11.21\%$
    • B) $88.79\%$
    • C) $50.00\%$
    • D) $18.02\%$

    Answer: B) $88.79\%$

  2. Which of the following statements about mass percentage is correct?

    • A) It represents the percentage by volume of an element in a compound.
    • B) It is calculated by dividing the mass of the compound by the mass of the element.
    • C) It indicates the proportion of an element's mass relative to the total mass of the compound.
    • D) It is always $100\%$ for any element in a pure compound.

    Answer: C) It indicates the proportion of an element's mass relative to the total mass of the compound.

  3. A reaction has a theoretical yield of $25.0 \text{ g}$. If the actual yield obtained is $20.0 \text{ g}$, what is the percent yield?

    • A) $125\%$
    • B) $80.0\%$
    • C) $20.0\%$
    • D) $5.0\%$

    Answer: B) $80.0\%$

  4. A percent yield greater than $100\%$ in a chemical reaction most likely indicates:

    • A) A highly efficient reaction.
    • B) The reaction went to completion.
    • C) Impurities in the product or incomplete drying.
    • D) The actual yield was less than the theoretical yield.

    Answer: C) Impurities in the product or incomplete drying.

  5. How many molecules are present in $0.50 \text{ mol}$ of carbon dioxide ($CO_2$)?

    • A) $6.022 \times 10^{23}$ molecules
    • B) $3.011 \times 10^{23}$ molecules
    • C) $1.204 \times 10^{24}$ molecules
    • D) $0.50$ molecules

    Answer: B) $3.011 \times 10^{23}$ molecules

  6. A sample contains $1.8066 \times 10^{24}$ atoms of iron. How many moles of iron are present?

    • A) $0.300 \text{ mol}$
    • B) $1.00 \text{ mol}$
    • C) $3.00 \text{ mol}$
    • D) $6.022 \times 10^{23} \text{ mol}$

    Answer: C) $3.00 \text{ mol}$

  7. How many oxygen atoms are present in $22.0 \text{ g}$ of carbon dioxide ($CO_2$)? (Atomic weights: $C = 12.011 \text{ g/mol}$, $O = 15.999 \text{ g/mol}$)

    • A) $3.011 \times 10^{23}$ atoms
    • B) $6.022 \times 10^{23}$ atoms
    • C) $1.204 \times 10^{24}$ atoms
    • D) $1.0 \text{ mol}$

    Answer: B) $6.022 \times 10^{23}$ atoms

  8. If the actual yield of a reaction is $10.0 \text{ g}$ and the percent yield is $80.0\%$, what is the theoretical yield?

    • A) $8.0 \text{ g}$
    • B) $12.5 \text{ g}$
    • C) $10.0 \text{ g}$
    • D) $120.0 \text{ g}$

    Answer: B) $12.5 \text{ g}$

  9. Avogadro's number ($N_A$) represents:

    • A) The number of grams in one mole of any substance.
    • B) The volume occupied by one mole of any gas at STP.
    • C) The number of particles (atoms, molecules, ions) in one mole of any substance.
    • D) The mass of a single atom of carbon-12.

    Answer: C) The number of particles (atoms, molecules, ions) in one mole of any substance.

  10. A compound has a formula weight of $180.16 \text{ g/mol}$. If it contains $40.00\%$ carbon by mass, and the atomic weight of carbon is $12.011 \text{ g/mol}$, how many carbon atoms are in one molecule of the compound?

    • A) $3$
    • B) $4$
    • C) $6$
    • D) $12$

    Answer: C) $6$

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