Chemistry 2000, Fall 96, Assignment 2

Due: October 10

1. Consider the phase diagram of carbon:

   

   The curve separating the diamond and graphite regions intersects the P axis at and the curve between the graphite and vapour regions intersects the T axis at .

   1. It is often cheaper to manufacture industrial diamonds (to be used in cutting tools, for instance) than to mine them. Is it easier to make diamonds at higher or lower temperatures? Explain how you get this information from the phase diagram. [4 marks]
   2. Does the melting temperature of diamond increase or decrease with pressure? Explain how you can tell from the phase diagram. [4 marks]
   3. What phase change eventually occurs when graphite is heated at 1atm? (Hint: Carefully consider the scale of the P axis.) [2 marks]
2. 1. Explain how you can make water boil at room temperature. Hint: Refer to the phase diagram of water, Fig. 11.49 of your textbook. [5 marks]
   2. Explain the physical principles behind overnight dew formation. [5 marks]
3. While working in an industrial laboratory, you are asked to analyze a liquid sample which contains a mixture of hexane ( ) and cyclohexane ( ). The vapour pressure of the sample is 140mmHg at 298K. The vapour pressures of pure hexane and cyclohexane at this temperature are, respectively, 148 and 110mmHg. What is the percentage (by mass) of cyclohexane in the mixture? [10 marks]
4. In industry, water is often used as a coolant by running uninsulated pipes through the factory (where the water picks up heat) and then outside (where the water can release heat to the cooler air). The plant engineer comes to your office and tells you that he has been having trouble with the cooling pipes freezing in the winter. By his reckoning, the temperature in the pipes should never drop below C, provided you can keep the water moving. The cooling system contains 18t (1 tonne is 1000kg) of water. What mass of sodium chloride would you have to add to the cooling system to keep the pipes from freezing? [10 marks]
5. Your highly varied career then takes you to the police forensic lab. One night (a dark and stormy one, no doubt), a police officer brings in a sample of a substance which she believes to be 3,4-methylenedioxyamphetamine (MDA, one of the many varieties of the street drug ``ecstasy''). The molecular formula of MDA is . Your supervisor just bought you a shiny new osmometer which you are itching to try out so you decide to determine the molecular weight of the unknown substance as a first step toward working out its structure. You dissolve 2.841mg of the sample in 100mL of water. The osmotic pressure of this solution at is 394Pa. Compute the molar mass of the unknown. Is it likely to be MDA? [10 marks]