Chemistry 2000, Fall 1996 Final examination

You have three hours to complete this exam. You may not leave the examination hall in the first hour or in the last 10 minutes. The aggregate value of all questions on this exam is 100. Your answers must be recorded in the exam booklets provided. Please note the useful information at the end of the paper.

1. If a chemical compound which is destroyed by a first-order reaction has a half-life of 18s, how long would it take before less than 0.1% of the original quantity remains? [5 marks]
2. The rate of formation of a product P in a chemical reaction is measured as a function of the concentration of a reactant R and the following results are obtained:

   

   What is the rate law for this reaction? [4 marks]

3. What is the pH of a solution made by dissolving 100g of potassium hydroxide in 500mL of water at 298K? The molar mass of KOH is 56.1056g/mol. [8 marks]
4. Would the solubility of calcium hydroxide be higher in a pH 4 buffer or in a pH 10 buffer? Explain briefly. [3 marks]
5. The enthalpy of formation of liquid boron trichloride is at 298K. The enthalpy of vaporization of boron trichloride at this temperature is 23kJ/mol. What is the enthalpy of formation of gaseous ? Hint: Start by writing down the reactions to which the various enthalpies refer. [4 marks]
6. Calculate the equilibrium constant for the reaction

   

   at 298K. [4 marks]

7. Ozone ( ) reacts with molecular bromine in basic solution. The products are molecular oxygen and bromate ions ( ). Balance the reaction. [6 marks]
8. A sugar is isolated from a natural source. A solution of the purified sugar is prepared by dissolving 10.00g of it in water in a 100.00mL volumetric flask. The osmotic pressure of the solution at is 719kPa. What is the molar mass of the sugar? [10 marks]
9. 3.5g of potassium nitrate (molar mass 101.1032g/mol) is dissolved in 100g of water initially at . Calculate the final temperature of the solution assuming that no heat is lost to the surroundings. The specific heat capacity of water is . Aqueous ions have a negligible heat capacity. [10 marks]
10. 15g of sodium dihydrogen phosphate ( , molar mass 119.98g/mol) is dissolved in 150mL of water. What is the pH of the solution? The of the first proton of the dihydrogen phosphate anion is 7.2 at 298K. [10 marks]
11. The solubility product of silver sulfate ( ) is at 298K. What is the solubility of this compound, expressed in g/L? The molar mass of silver sulfate is 203.97g/mol. [10 marks]
12. Suppose that you want to generate an electrical current from the following half-reactions:

    

    

    The standard reduction potentials are given for a temperature of 298K.

    1. Describe the apparatus you would need to obtain electricity from these reactions. A carefully labeled sketch is adequate. [6 marks]
    2. Suppose that a coworker has prepared the following solutions:

       

       What is the spontaneous overall reaction which occurs in the apparatus you described above and what voltage is produced at 298K? [10 marks]

13. For the reaction

    

    at 298K.

    1. Calculate the equilibrium constant for this reaction. [6 marks]
    2. If we start with 10L of a 2.5mol/L solution of chromium (III), what mass of solid chromium is produced if an excess of manganese is tossed into the solution? Chromium metal has a molar mass of 51.996g/mol. [4 marks]

Useful formulas

For a first-order reaction, and . For a Brønsted acid, is the equilibrium constant for the reaction . For a Lewis acid, it is the equilibrium constant for the reaction . (the solubility product) is the equilibrium constant for the dissociation of an ionic solid into aqueous ions. or

Useful data

To convert degrees Celcius to Kelvin, add 273.15.

at