The Foundations of Chemistry

We convert 8.92 torr to atmospheres to be consistent with the units of R.

n

RT

V

1.43^10 ^5 mol pepsin

Thus, 0.500 g of pepsin is 1.43 10 ^5 mol. We now estimate its molecular weight.

_?_ g/mol

1.43

0



.5

1

0

0

0



g

 (^5) mol 3.50^10
(^4) g/mol
The molecular weight of pepsin is approximately 35,000 amu. This is typical for medium-sized
proteins.
You should now work Exercise 90.
8.92 torr 76
1
0
at
t
m
orr
(0.0300 L)

0.0821m
L
o
a
l
t

m
K
(300 K)
Living cells contain solutions. When
living cells are put in contact with
solutions having different total solute
concentrations, the resulting osmotic
pressure can cause solvent to flow
into (left) or out of (right) the cells.
The freezing point depression of this
very dilute solution would be only
about 0.0009°C, which would be
difficult to measure accurately. The
osmotic pressure of 8.92 torr, on the
other hand, is easily measured.
Problem-Solving Tip:Units in Osmotic Pressure Calculations
Strictly speaking, the equation for osmotic pressure is presented in terms of molarity,
MRT.Osmotic pressure, , has the units of pressure (atmospheres); M(mol/L); and
T(kelvins). Therefore, the appropriate value of Ris 0.0821 
m
L
o
a
l
t

m
K
. We can balance the
units in this equation as follows.
atm
m
L
ol

m
L
o
a
l
t

m
K
(K)
When we use the approximation that M
m,we might think that the units do not
balance. For very dilute aqueous solutions, we can think of Mas being numericallyequal
to m,but having the units mol/L, as shown above.
Cells expand in solution of
lower solute concentration
(a hypotonic solution)
Normal cells in isotonic solution Cells shrink in solution of
greater solute concentration
(a hypertonic solution)