Final Exam Review Problem 2 & 3 KEY

#2.

25 grams of pentane (C₅H₁₂) combusts. Find the mass of the liquid product produced.

First of all, you need to recognize that combustion is burning. Combustion must occur in the presence of oxygen. And, the products of combustion include water and carbon dioxide.

1 C₅H₁₂ + 8 O₂ --> 5 CO₂ + 6 H₂O

25 g C₅H₁₂    x    1 mole C₅H₁₂    x    6 moles H₂O x    18.015 g H₂O
                           72.15 g C₅H₁₂          1 mole C₅H₁₂        1 mole H₂O

= 37.45 g --> 37 g (2 SF) of water is produced

#3

What is the molarity of a solution of nitric acid if 65.3 mL reacts completely with 22.7 mL of a 1.6 M potassium hydroxide solution?

This is a double replacement reaction and stoichiometry problem using molarity.

1 HNO₃ + 1 KOH --> 1 KNO₃ + 1 H₂O
? M 1.6 M
65.3 mL 22.7 mL

22.7 mL    x    1L    x    1.6 mole KOH   x   1 mole HNO₃ = .036 moles HNO₃
                      10³mL              1 L                 1 mole KOH

.036 moles   x   10³ mL = .56 M (2 SF)
65.3 mL            1 L