PChem I     Exam 3     8 November 1999

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  1. You don't buy used cars from northern states because those states use NaCl on the roadways in winter which promotes rusting of important things, like brake lines! Almost as cheap per kg as NaCl are epsom salts, MgSO4·7H2O, and they are far more benign on auto undercarriages. Better still, plants need the Mg for their chlorophyll while NaCl kills plants. But they differ in solubility in cold water. NaCl's solubility is 357 g/L while epsom is 710 g/L. Which will melt ice in the colder climate? And by how much colder (°C)? Kf = 1.86 K mol-1 kg(H2O)

  2. When the U.S. minted silver coins, they were 90:10 Ag/Cu by weight. If Tf(Ag) = 1234 K and DfH(Ag) = 11.3 kJ/mol, what should the melting point of silver coins have been? It was actually 890°C. What can you say about the ideality of a Ag:Cu mixture? (Yes, we're treating Ag as the solute even though it's at 90% abundance.)

  3. Find DS for the base reaction of aqueous ammonia from the following table of Kb as a function of T (HCP, p. 1743, 1958):

    T(°C)Kb
    201.710×10-5
    251.774×10-5
    301.820×10-5

  4. While the slope of pH vs Vtitrant is extremely high as it passes through Vequivalence, it is rather low as it passes through V½equivalence (or buffers wouldn't be useful). For the relatively easy case of titrating a weak acid with a strong base of exactly the same concentration, C, find (dpH/dVtitrant) at ½ equivalence as a function of the original base volume V0 by direct differentiation of the Henderson-Hasselbalch equation. (Of course, for equal concentrations, V½equivalence = ½V0.)

    Life will be simpler if you use Vtitrant = x + ½V0 since not only is x=0 at ½ equivalence but also dx = dVtitrant. [And log(x) = 0.434 ln(x) is useful.]

  5. The mean activity coefficient, g±, in a 0.100 mol kg-1 CaCl2(aq) solution is 0.524 at 25°C. What is the percentage error in the value predicted by the Debye-Hückel limiting law? (Atkins E10.10b)

  6. Tables of reduction potentials are written with H+ or OH- prominently included since many reactants go to different products in acid or base solution. For example, in acid, MnO4- reduces to Mn2+, but in base, it reduces to MnO2, not only changing the stoichiometry but also the number of electrons transferred, n.

    Suppose, however, that a reduction reaction is found that proceeds to the same products regardless of whether the reduction is run in acid or base. Then it would have a standard potential of either E°acid or E°base. If n is the (same) number of electrons transferred, and n is the magic stoichiometric coefficent of H+ in the acidic version of the reduction (n positive if H+ is a product but negative otherwise), find the expression that relates E°acid and E°base. (Ignore activity coefficients.)

Last modified 14 November 1999