7 . The membrane must be permeable to water, but not to salt. If it were permeable to salt, the
salt would diffuse across the membrane into the freshwater container (down its concentration
gradient) until the molarities of the two containers were the same. However, the volume of the
salt water container increased, indicating that fresh water diffused across the membrane from
a region of low solute concentration to one of high solute concentration. The water level rose
until it exerted enough pressure to counterbalance the tendency to diffuse; this pressure is
known as the osmotic pressure.8 . B
Using the formula Π = MRT, we see that osmotic pressure and temperature are directly
proportional; i.e., if temperature increases, osmotic pressure will also increase. Thus, choice B
is the correct answer.9 . –4.65
This question applies the concept of freezing-point depression. If 0.5 mol of a nonelectrolyte
solute such as glucose is dissolved in 200 g of H 2 O, then the molality of the solution is:0.5 mol/0.200 kg solvent = 2.5 m
Using the equation ΔTf = Kf m, the freezing-point depression is 2.5 × 1.86°C, or 4.65°C, and the
new freezing point is thus the original freezing point –4.65°C = (0 – 4.65)°C = –4.65°C.10 . C
The osmotic pressure (Π) of a solution is given by Π = MRT. At STP, T = 273 K. The formula
weight of NaCl is 58.5. The number of moles in the solution described is:
117 g/(58.5 g/mol) = 2 moles of NaCl
But since NaCl is a strong electrolyte, it dissociates in aqueous solution and there are actually 4
moles of particles present in the solution, i.e., 2 moles of Na+ and 2 moles of Cl–. The volume of
the solution is 1 L, and so the total molarity of solutes is 4 M.