CHEM 123

Sample Exam

Stoichiometry and Thermochemistry


1. The combustion reaction of PH3 is shown by the equation


PH3(g) + 2O2(g) --> H3PO4(s).


a. How many grams of H3PO4 will be formed from the combustion of 85.0 g of PH3 in excess O2?

b. If 155 g of H3PO4 are isolated, calculate the % yield.

2. Ammonia gas can be recovered from NH4Cl by the reaction:


CaO(s) + 2NH4Cl(s) --> CaCl2(s) + H2O(g) + 2NH3(g).


a What mass of NH3 can be obtained from the reaction of 75.0 g of NH4Cl with 50.0 g of CaO?


b. What mass of excess reactant will be left over at the end of the reaction?

3a. Calculate the molarity of a solution of CaCl2 that contains 0.185 g of solute in 475 mL of solution.

b. Calculate the molarity of Ca2+ and Cl- in the solution above.


4. How many grams of BaSO4 can be precipitated from solution by adding excess NaSO4 to 75.0 mL of 0.355 M BaCl2 solution? The balanced equation is


BaCl2(aq) + Na2SO4(aq) --> BaSO4(aq) + 2NaCl(aq)


5. In an acid-base reaction, 10.00 mL of 0.0911 M H2SO4 are neutralized by 22.34 mL of NaOH solution. What is the molarity of the NaOH solution?

6. Calculate the enthalpy of reaction for the synthesis of hydrogen chloride gas


H2(g) + Cl2(g) --> 2HCl(g)


from the following data:


NH3(g) + HCl(g) --> NH4Cl(s) Ho = -176.0 kJ


N2(g) + 3H2(g) --> 2NH3(g) Ho = -92.22kJ


N2(g) + 4H2(g) + Cl2(g) --> 2NH4Cl(s) Ho = -628.86 kJ

b. Is the reaction endothermic or exothermic as written? Explain.


7. How many grams of ice at 0°C are necessary to cool 1 gallon of lemonade (3.8 kg) from 18°C to 3°C? Assume that the specific heat of lemonade is the same as that of water. The enthalpy of fusion for ice is 333 J/g.

8. Calculate the enthapy of formation of C2H8 from bond energies. The structure of C2H8 is given below:






Bond energies: C-C 347 kJ/mol

C-H 463

H-H 435