Practice question

Demonstration using U-tube separated by membrane permeable only to water nicely illustrates colligative water movement. Osmotic pressure approximated by van't Hoff law pi equals i times C times R times T proportional to total particle concentration. Solution A holding 4 molar NaCl dissociates into sodium and chloride giving factor near two, so effective osmolarity roughly 8 osmoles, solution B 10 molar near 20 osmoles, markedly higher. Chemical potential of water mu equals mu naught plus RT ln a water minus V bar P, and a water declines with solute. Therefore water potential higher in dilute arm and lower in concentrated arm. Spontaneous flux proceeds from high potential to low potential, meaning from dilute 4 molar side toward concentrated 10 molar side, attempting to equalize activities. Volume rises on high solute side, falls on low solute side until hydrostatic pressure difference compensates. Sodium chloride itself does not cross because membrane excludes ions. Analogous processes regulate cell volume. Hence in this scenario water moves from Side A containing 4M NaCl to Side B containing 10M NaCl following osmotic gradient toward higher solute concentration.