In colligative property measurements, the freezing point changes by approximately how many degrees Celsius per osmole increase of solute?

• A. 0.3 C decrease in vapor pressure per osmole
• B. 1.86 C decrease in freezing point per osmole
• C. 0.52 C increase in boiling point per osmole
• D. 1.7 x 10^4 C increase in osmotic pressure per osmole

Answer: (B)

In colligative properties, how much the freezing point changes depends on how many dissolved particles are present, not on their identity. The freezing point depression is given by ΔTf = i Kf m. For water, Kf is about 1.86 °C·kg/mol. So, for one osmole of solute in 1 kg of solvent (m ≈ 1) and with an ideal solute (i ≈ 1), the freezing point decreases by roughly 1.86 °C. If the solute dissociates, the effective number of particles (i) increases, making the depression larger accordingly. The other options refer to different colligative properties: vapor pressure changes, boiling point elevation (≈0.52 °C per osmole), or osmotic pressure, none of which describe the freezing-point change per osmole.