Practice Problems

 Question: The combustion of methane is best represented by the equation: A. CH4(g) + 2 O2(g) -> CO2(g) + 2 H2O(l) B. CO2(g) + 2 H2O(l) -> CH4(g) + 2 O2(g) C. CH4(g) + 3/2 O2(g) -> CO(g) + 2 H2O(l) D. some other equation Answer: A - Reactants are written to the left of the arrow. The equation shows the correct formulas and is balanced.

 Question: When water solutions of CaCl2 and Na2CO3 are mixed, a precipitate of calcium carbonate is formed. The equation for the reaction is: A. CaCl2 + Na2CO3 -> CaCO3 + 2 NaCl B. Ca(aq) + CO3(aq) -> CaCO3(s) C. CaCO3(s) -> Ca2+(aq) + CO32-(aq) D. Ca2+(aq) + CO32-(aq) -> CaCO3(s) Answer: D - Showing only those species directly involved.

 Question: Using the balanced equation for the preceding question, how many moles of solid precipitate for each mole of Ca2+ reacting? A. 0 B. 0.5 C. 1 D. 2 Answer: C - The understood "ones" for coefficients indicate the mole ratio: 1 mol Ca2+ : 1 mol Ca32- : 1 mol CaCO3.

 Question: Write a balanced chemical equation for each reaction: A. pure carbon is burned in air to give carbon dioxide B. carbon dioxide dissolves in water to give HCO3-(aq) and H+(aq) C. a precipitate of Ag2CrO4 forms when water solutions of AgNO3 and Na2CrO4 are mixed Answer: A - C(s) + O2(g) -> CO2(g) B - Note that all species have been given in the statement of the problem:      CO2(g) + H2O(l) -> HCO3-(aq) + H+(aq) C - c Ag+(aq) + CrO42-(aq) -> Ag2CrO4(s)

 Question: When the following equation is balanced, using the smallest whole number of coefficients,      ____H2S(g) + ____O2(g) -> ____SO2(g) + ____H2O(l) the coefficients for H2S and SO2 are, respectively, A. 1 and 1 B. 1 and 2 C. 2 and 1 D. the same Answer: D - Since S appears only in H2S and SO2 and with the same subscript, the coefficients must be the same. The coefficients are, left-to-right: 2, 3, 2, and 2.

 Question: Solid calcium carbonate, CaCO3, reacts with hydrochloric acid, H+(aq), to give Ca2+, CO2(g) and H2O. (Chloride ion remains in solution.) A. Write a balanced equation for the reaction. B. What is the concentration of the acid (M) if 10.0 g CaCO3 (formula mass = 100) requires the use of 167 cm3 of acid? Answer: A - CaCO3(s) + 2 H+(aq) -> Ca2+(aq) + CO2(g) + H2O(l) B - concentration of acid = mols H+/lires (or dm3) solution    - moles acid = 10.0 g CaCo3 x (1 mol CaCo3 / 100 g CaCo3) x (2 mol H+ / mol CaCo3) = 0.200 mol H+    - concentration acid = 0.200 mol/0.167 l (dm3) = 1.20 M

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