Chemistry 2720, Fall 1995 Final Examination

Section 1

Answer all questions in this section, with an aggregate value of 27 marks.

1. Balance the following redox reactions:
   1. Permanganate (MnO ) reacts with sulphur dioxide (SO ) to form manganese(II) ions and HSO in acidic solution.
   2. Solid bismuth(III) hydroxide (Bi(OH) ) reacts with the SnO anion to form SnO and metallic bismuth in basic solution.
2. The solubility of gases in water generally decreases with increasing temperature. Explain this observation using thermodynamics.
3. A fuel cell operates on liquid ethanol at 298K. Oxygen is supplied at a constant pressure of 0.21atm and carbon dioxide is removed at such a rate that its pressure is an essentially constant 0.1atm.
   1. What voltage does this fuel cell generate?
   2. If the cell is improperly operated, it is also possible for it to generate carbon monoxide instead of carbon dioxide. Assuming that the carbon monoxide pressure is 0.1atm and that the cell otherwise operates under identical conditions, what is the operating voltage?
4. Mercury is used to make dental filling amalgam. When it is first made, dental amalgam is essentially a solution of silver and tin in liquid mercury.
   1. Assume that the solution contains a ratio of Hg:Ag:Sn of 24:15:5 and calculate the equilibrium vapour pressure of mercury over this solution at C.
   2. Assume that the mercury vapour behaves as an ideal gas. The maximum short-term occupational exposure to mercury allowed is 0.1mg/m . According to your calculation, are traditional mercury amalgam dental fillings safe? The molar mass of mercury is 200.59g/mol.
5. 3.20mg of a protein of unknown structure is dissolved in 100g of water. The osmotic pressure of this solution causes a capillary rise of 1.3mm of the solution when a measurement is made against pure water at 298K. What is the molar mass of this protein?
6. The solubility product of calcium fluoride is . What is the solubility of calcium fluoride in a 1.5mol/kg sodium thiocyanate (NaSCN) solution at 298K? Do a full Debye-Hückel calculation and note that calcium thiocyanate is very soluble.

Section 2

Answer any two questions in this section. Do not answer more than two questions. This section is worth 18 marks.

1. How much copper(I) iodide can be dissolved in a saturated copper(I) chloride solution at 298K? Do a full Debye-Hückel calculation. Hint: Calculate the 's first.
2. 1. What is the entropy of vaporization of mercury at 298K?
   2. What is the boiling temperature of mercury?
   3. What is the entropy of vaporization of mercury at its boiling point?
3. The electrochemical cell

   

   is operated at 298K with a concentration of sodium bromide in the left half-cell of 0.1mol/kg and a concentration of silver sulphate in the right half-cell of mol/kg. Reduction occurs in the right half-cell with an observed output voltage of 0.496V. Considering the effect of nonideality of the ionic solutions, compute the solubility product of silver bromide. Useful fact: The left half-cell reaction is

   

4. When 10g of sodium nitrate ( , molar mass 84.99g/mol) is dissolved in 200g of water, it completely dissociates and the temperature of the water drops by C. When 10g of nitric acid (HNO, molar mass 63.01g/mol) is dissolved in 200g of water, 93% of the acid dissociates to nitrate and an aqueous proton and the temperature of the water increases by C. The enthalpy of formation of nitric acid is known from other experiments to be -174.1kJ/mol. Assume that the dissociation of the acid is responsible for most of the temperature rise in the latter case. What are the enthalpies of formation of sodium nitrate and of the nitrate anion?

Useful data

R = 8.31441JK mol .

C/mol.

To convert degrees Celcius to Kelvins, add 273.15.

To convert atmospheres to Pascals, multiply by 101325.

Density of water at 298K: 997.07kg/m

Heat capacity of liquid water: 4.184Jg K