Questions about Ch 13 EOC exercises

[Zellmer, Robert]

I've received several questions about the same end-of-chapter exercises.
Here's partial answers to those questions to get your started on them if
you haven't tried them or are still having problems with them. This
seemed to be most peoples problems with them, not knowing how
to start them.

13.57:
This is about an alloy of 80% Cu and 20% Zn.  The primary question
was how to start this.  I would say to do this how I suggested.  You
have a solid solution.  You're given % composition.  Assume 100 g
of soln (metal).  That means you have 80 g Cu and 20 g Zn.  This
is what you would have done in 1210 to determine an empirical formula.
Then use the density of the soln (metal) to convert 100 g sol to volume
of soln.  By the way, 1 m3 = 103 L (similar to 1 cm3 = 1 mL).

13.60:
The primary problem I've seen with this one is people not knowing for
gases a volume % is the same as a mole % at a constant P and T.  Thus a
mixture with a vol % of 4 % CO2 has a mole % of 4 % CO2.  Also,
mole % = mole fraction * 100.

I've also seen people having trouble remembering that for a gaseous mixture
the relationship between partial pressure of a gas and total pressure.  The
partial pressure of gas A is given by the following,

            P_A = X_A * P_total

You need this for VP problems to calculate the mole fraction of a substance in
the gas phase above a soln.

13.98
This question is a conc. conversion question.  You're given a 1.80 M LiBr soln
in the solvent acetonitrile (CH3CN) and the density of the soln. You're then
asked to convert this to molality, mole fraction of LiBr and mass % (of CH3CN).

I approached this type of problem in class by listing what the given conc means
first and then assuming what's in the denominator,

                             1.80 mol LiBr
    1.80 M LiBr =   -------------------
                                  1 L soln

Assume you have a beaker with 1 L soln (what's in the denominator). That then
also gives you what's in the numerator, 1.80 mol LiBr.

Then for each conc. you're trying to get write down it's definition and then
determine how to get to that conc. unit based on what you have.

Don't forget, to from molarity to any of the others (and vice versa) you will need
density of the soln to convert from L soln to mass of soln (or vice versa).

Hopefully this helps with any lingering questions.

Dr. Zellmer

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