Problems A
335
Thus, according to these measurements, the apparent degree of dissociation
for one-molal NaCl is 0.88, or 88.0%.
If we have an electrolyte such as CaCl 2 , we get a somewhat different expression. Suppose we add 0.50 moles to 1 kg of water, and it behaves as if a is the
fraction of the molecules that dissociate. In solution we would have
0.50(1 - a) moles of CaCl 2
O.SOa moles of Ca
2+
2 x 0.50o! moles of Cl~
m = 0.50(1 + 2a) total moles/kg of water
From this, and the measured freezing-point depression (2.63°C), we can evaluate a as we did for NaCl and find it to be 0.914.
When you determine the value of a for a weak electrolyte, such as acetic
acid or ammonia, the result must be interpreted in a different manner. In such a
situation a really does correspond to the degree of dissociation, for most of the
molecules actually are present in the undissociated form. The problems are
worked in exactly the same way as for strong electrolytes; it is only the interpretation that is different.
PROBLEMS A
1. A solution is prepared by dissolving 1.28 g of naphthalene, Cj 0 H 8 , in 10.0 g of
benzene.
(a) What is the lowering of the freezing point of benzene?
(b) What is the freezing point of the solution?
(c) What is the mole fraction of benzene in the solution?
(d) The vapor pressure of pure benzene is 100 torr at room temperature. What
is the vapor pressure of this solution at the same temperature?
(e) What is the boiling point of the solution?
2. A student uses a thermometer on which she can read temperatures to the
nearest 0.1°. In a laboratory experiment, she observes a freezing point of
5.4°C for pure benzene. She then dissolves 0.75 g of an unknown in 15.0 g of
benzene and finds the freezing point of the solution to be 2.8°C. What is the
molecular weight of the unknown?
3. What will be the freezing point of a 10.0% solution of sucrose, C^F^O,,, in
water?
4. A 10.0 g sample of naphthalene (C 10 H 8 ) mothballs is added to 50.0 ml of
benzene (density = 0.879 g/ml). What will be the boiling point of this solution?
5. Expensive special thermometers are usually needed for determinations of
freezing-point depression and boiling-point elevation but, when camphor is
used, a common lab thermometer may be employed. A student mixes 0.1032 g
335
Thus, according to these measurements, the apparent degree of dissociation
for one-molal NaCl is 0.88, or 88.0%.
If we have an electrolyte such as CaCl 2 , we get a somewhat different expression. Suppose we add 0.50 moles to 1 kg of water, and it behaves as if a is the
fraction of the molecules that dissociate. In solution we would have
0.50(1 - a) moles of CaCl 2
O.SOa moles of Ca
2+
2 x 0.50o! moles of Cl~
m = 0.50(1 + 2a) total moles/kg of water
From this, and the measured freezing-point depression (2.63°C), we can evaluate a as we did for NaCl and find it to be 0.914.
When you determine the value of a for a weak electrolyte, such as acetic
acid or ammonia, the result must be interpreted in a different manner. In such a
situation a really does correspond to the degree of dissociation, for most of the
molecules actually are present in the undissociated form. The problems are
worked in exactly the same way as for strong electrolytes; it is only the interpretation that is different.
PROBLEMS A
1. A solution is prepared by dissolving 1.28 g of naphthalene, Cj 0 H 8 , in 10.0 g of
benzene.
(a) What is the lowering of the freezing point of benzene?
(b) What is the freezing point of the solution?
(c) What is the mole fraction of benzene in the solution?
(d) The vapor pressure of pure benzene is 100 torr at room temperature. What
is the vapor pressure of this solution at the same temperature?
(e) What is the boiling point of the solution?
2. A student uses a thermometer on which she can read temperatures to the
nearest 0.1°. In a laboratory experiment, she observes a freezing point of
5.4°C for pure benzene. She then dissolves 0.75 g of an unknown in 15.0 g of
benzene and finds the freezing point of the solution to be 2.8°C. What is the
molecular weight of the unknown?
3. What will be the freezing point of a 10.0% solution of sucrose, C^F^O,,, in
water?
4. A 10.0 g sample of naphthalene (C 10 H 8 ) mothballs is added to 50.0 ml of
benzene (density = 0.879 g/ml). What will be the boiling point of this solution?
5. Expensive special thermometers are usually needed for determinations of
freezing-point depression and boiling-point elevation but, when camphor is
used, a common lab thermometer may be employed. A student mixes 0.1032 g