Practice Problems: Stoichiometry (Answer Key)

  1. Balance the following chemical reactions:

    a. 2 CO + O2 2 CO2

    b. 2 KNO3 2 KNO2 + O2

    c. 2 O3 3 O2

    d. NH4NO3 N2O + 2 H2O

    e. 4 CH3NH2 + 9 O2 4 CO2 + 10 H2O + 2 N2

    f. Cr(OH)3 + 3 HClO4 Cr(ClO4)3 + 3 H2O

  2. Write the balanced chemical equations of each reaction:

    a. Calcium carbide (CaC2) reacts with water to form calcium hydroxide (Ca(OH)2) and acetylene gas (C2H2).

    CaC2 + 2 H2O C2H2 + Ca(OH)2

    b. When potassium chlorate (KClO3) is heated, it decomposes to form KCl and oxygen gas (O2).

    2 KClO3 2 KCl + 3 O2

    c. C6H6 combusts in air.

    2 C6H6 + 15 O2 12 CO2 + 6 H2O

    d. C5H12O combusts in air.

    2 C5H12O + 15 O2 10 CO2 + 12 H2O

  3. Given the following reaction: Na2S2O3 + AgBr NaBr + Na3[Ag(S2O3)2]

    a. How many moles of Na2S2O3 are needed to react completely with 42.7 g of AgBr? 0.455 mol Na2S2O3

    b. What is the mass of NaBr that will be produced from 42.7 g of AgBr? 23.4 g NaBr

  4. From the reaction: B2H6 + O2 HBO2 + H2O

    a. What mass of O2 will be needed to burn 36.1 g of B2H6? 125 g O2

    b. How many moles of water are produced from 19.2 g of B2H6? 1.39 mol H2O

  5. Calculate the mass (in kg) of water produced from the combustion of 1.0 gallon (3.8 L) of gasoline (C8H18). The density of gasoline is 0.79 g/mL.

    4.3 kg H2O

  6. One mole of aspartame (C14H18N2O5) reacts with two moles of water to produce one mole of aspartic acid (C4H7NO4), one mole of methanol (CH3OH) and one mole of phenylalanine.

    a. What is the molecular formula of phenylalanine? C9H11NO2

    b. What mass of phenylalanine is produced from 378 g of aspartame? 212 g phenylalanine

  7. KO2 is used in a closed-system breathing apparatus. It removes carbon dioxide and water from exhaled air. The reaction for the removal of water is: KO2 + H2O O2 + KOH. The KOH produced is used to remove carbon dioxide by the following reaction: KOH + CO2 KHCO3.

    a. What mass of KO2 produces 235 g of O2? 696 g KO2

    b. What mass of CO2 can be removed by 123 g of KO2? 76.1 g CO2

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