332
Colllgatlve Properties
PROBLEM:
Compound Y of the preceding problem is found by analysis to have the composition
C = 49.0%,
H = 2.7%,
Cl = 48.3%
Find the formula and the exact molecular weight.
SOLUTION:
Using the methods of Chapter 10, we find the empirical formula to be C 3 H 2 C1. If
this were the true formula, the molecular weight would be 73.5. The freezing point
depression gives a molecular weight of approximately 150. This is not an accurate
value, for the experimental measurement is subject to some error, but it indicates
that the true molecular weight is near 150.
Knowing the empirical formula to be C 3 H 2 C1, we know that the real formula is
(C 3 H 2 Cl) n , where n is some small integer. If n is 1, a mole is 73.5 g; if n is 2, a mole
is 147 g; ifn is 3, a mole is 221 g. Our experimental molecular weight of 150 enables
us to decide that the true value of n is 2, because 147 is the molecular weight
nearest our experimental value. Thus the formula is (C 3 H 2 C1) 2 , or C 6 H 4 C1 2 .
OSMOTIC PRESSURE
When solvent and solution are separated by a semipermeable membrane that
permits solvent molecules to pass, an osmotic pressure is developed in the
solution. This pressure, ir, is defined as the mechanical pressure that must be
applied to the solution to prevent solvent molecules from diffusing into it. For
water solutions the relationship between IT and the molal concentration m is
given by the equation
TT = (0.0821 Dm
(21-4)
where IT is in atmospheres, and T is the absolute temperature. Note that Equation 21-4 is just like Equations 21-2 and 21-3, except that the constant multiplied
by m is temperature-dependent. The only common practical solvent for osmosis
is water, so the only constant we shall consider is 0.0821. A one molal solution
at 0.0°C would have an osmotic pressure of
TT = (0.0821)(273)( 1.00) = 22.4 atm
In comparison with the relatively small vapor pressure lowering caused by
relatively concentrated solutions, the osmostic effect is gigantic. For example, a
1.00 x 10~
4 molal solution at 25.0°C would have an osmotic pressure of
Colllgatlve Properties
PROBLEM:
Compound Y of the preceding problem is found by analysis to have the composition
C = 49.0%,
H = 2.7%,
Cl = 48.3%
Find the formula and the exact molecular weight.
SOLUTION:
Using the methods of Chapter 10, we find the empirical formula to be C 3 H 2 C1. If
this were the true formula, the molecular weight would be 73.5. The freezing point
depression gives a molecular weight of approximately 150. This is not an accurate
value, for the experimental measurement is subject to some error, but it indicates
that the true molecular weight is near 150.
Knowing the empirical formula to be C 3 H 2 C1, we know that the real formula is
(C 3 H 2 Cl) n , where n is some small integer. If n is 1, a mole is 73.5 g; if n is 2, a mole
is 147 g; ifn is 3, a mole is 221 g. Our experimental molecular weight of 150 enables
us to decide that the true value of n is 2, because 147 is the molecular weight
nearest our experimental value. Thus the formula is (C 3 H 2 C1) 2 , or C 6 H 4 C1 2 .
OSMOTIC PRESSURE
When solvent and solution are separated by a semipermeable membrane that
permits solvent molecules to pass, an osmotic pressure is developed in the
solution. This pressure, ir, is defined as the mechanical pressure that must be
applied to the solution to prevent solvent molecules from diffusing into it. For
water solutions the relationship between IT and the molal concentration m is
given by the equation
TT = (0.0821 Dm
(21-4)
where IT is in atmospheres, and T is the absolute temperature. Note that Equation 21-4 is just like Equations 21-2 and 21-3, except that the constant multiplied
by m is temperature-dependent. The only common practical solvent for osmosis
is water, so the only constant we shall consider is 0.0821. A one molal solution
at 0.0°C would have an osmotic pressure of
TT = (0.0821)(273)( 1.00) = 22.4 atm
In comparison with the relatively small vapor pressure lowering caused by
relatively concentrated solutions, the osmostic effect is gigantic. For example, a
1.00 x 10~
4 molal solution at 25.0°C would have an osmotic pressure of