Page 92 - Basic physical chemistry for the atmospheric sciences

78 Basic physical chemistry

. Measured value of a colligative property
=
1 Expected value of a colligative property (4. 1 7)
The following exercise illustrates how van 't Hoff factors can be calcu­
lated and the reason for deviations from the expected values.
Exercise 4.6. A 1 M solution of NaCl in water causes an elevation in
l
the boiling point of . 04°C. What is the van't Hoff factor for NaCl?
Solution. From Eq. (4. 6 ) we see that a 1 M solution s expected to
i
produce an elevation in the boiling point of water of 0 . 52°C. Therefore,
from Eq. (4. 1 7 ) we have for NaCl
1 . 04
i = = 2 . 0
0.52
The value of i = 2 . 0 for NaCl suggests that its enhanced effect on
elevating the boiling point of water is due to its dissociation in water
into two ions: Na+(aq) and Cl - (aq) .
Exercise 4 . 7 . A 1 M aqueous solution of acetic acid, CH 3 COOH(l),
n
causes an elevation in the boiling point of . 540°C. What is the va ' t
0
fo
Hoff factor r C H 3COOH(I)?
Solution. Proceeding as in Exercise 4.6, and assuming that the value
m
of Kb in Eq. (4.6) is 0.520°C - 1 ,
0.540
i =
= .
0.520 l 04
Now, if an acetic acid molecule dissociates, it produces two ions:
H + (aq) and CH3Coo - ( aq) . Since the i value is only 1 . 04, this suggests
that only about 4% of the molecules of acetic acid dissociate in water.
This leads us to the subject of strong and weak electrolyte .
s

4 . 7 Strong and weak electrolytes; ion-product constant for water
If each molecule of a substance that dissolves in water dissociates into
ions (rather than the molecule remaining intact in the water), the
substance is called a strong electrolyte. Sodium chloride is a strong
electrolyte [see Reaction (4. 8 )] ; so is hydrochloric acid, HCl(g) , each
molecule of which when it enters water breaks up into two ions
HCl(g)� H + (aq) + C l - ( aq) (4 1 8)
.
Only one arrow is shown in Reaction (4. 1 8 ) , and it goes from left to
right; this indicates that the ionization of HCl goes essentially to
completion, and that the reverse reaction is negligible.