Chemistry 2000, Fall 96, Test 2

There are six questions.

1. Starch is a glucose polymer, i.e. a very large molecule made by stringing many glucose molecules together. The osmotic pressure of a starch solution is measured over a period of weeks. Between measurements, the solution is kept in a sealed container so that little evaporation takes place. It is observed that the osmotic pressure rises over time. Give a chemical interpretation of this phenomenon. [5 marks]
2. 1. Why can salt be used to melt ice? [3 marks]
   2. Ethanol is used as a deicing agent for airplane wings and in other applications where a solid deicing agent would be inconvenient. However, it is never used on roads. Can you suggest a reason why? Hint: One of your senses can be used to alert you to the presence of ethanol. [2 marks]
3. What is the vapour pressure of a solution made by dissolving 250g of sodium sulfite ( , molar mass 126.04g/mol) in 1.50L of water at 298K? The vapour pressure of pure water at this temperature is 23.756Torr (1Torr=1mmHg, in case you're curious) and the molar density of water is 55.33mol/L. [10 marks]
4. 1. What is the pH of a solution made by dissolving  10g of NaOH (molar mass 39.997g/mol) in 200mL of water at ? [5 marks]
   2. 100mL of 6.0mol/L HCl is added to the solution of question 4a. What is the pH after this addition? [5 marks]
5. 1. The 's of the three protons of phosphoric acid ( ) are 2.1, 7.2 and 12.3, all at . Do you think that acid phosphates could be used to make an effective pH 4 buffer? Why or why not? [2 marks]
   2. Suppose that you want to make a pH 12 buffer. You have bottles of phosphoric acid (molar mass 97.9951g/mol), sodium dihydrogen phosphate (119.9769g/mol), sodium hydrogen phosphate (141.9588g/mol) and sodium phosphate (163.9407g/mol). Which of these chemicals will you need to make the buffer? [2 marks]
   3. You want to make 10L of a pH 12 buffer with a total concentration of phosphates of 0.50mol/L. What masses of which of the above chemicals do you need to dissolve in the 10L of water to make the buffer? [8 marks]
6. Lead azide (molar mass 249.2g/mol) is an extremely dangerous explosive. The equilibrium constant for its disproportionation reaction

   

   at is .

   1. Describe the equilibrium state of a mixture of lead and lead azide left in an open container at under normal atmospheric conditions. The partial pressure of nitrogen in the atmosphere is 0.79atm. [2 marks]
   2. 1kg of lead azide comes to equilibrium in a 3L sealed box at . The box initially contains 1atm of pure nitrogen. What is the equilibrium nitrogen pressure in the box? [6 marks]