Question

Which of the following will have the lowest vapor pressure and which one will have the highest melting point? a) 0.75 molal C12H22O11(aq) b) 1.75 molal C12H22O11(aq) c) pure water

Answer 1

a) Given, 0.75 molal solutions of C12H22O11 (molar mass342.3 g/mol).

0.75 molal = 0.75 mole/kg = 0.75 mole/1L = 0.75M as solvent is water.

Similarly 1.75 molal solution = 1.75 M of solute

We know as per the colligative properties and C12H22O11 is a non-volatile solute. As a number of moles increase the vapor pressure decreases. Hence, solution b) 1.75 molal C12H22O11(aq) will have the lowest vapor pressure. Whereas, pure water will have the highest vapor pressure.

b) The melting point is nothing but the freezing point. As molality increases freezing point decreases. Hence, the solution b) 1.75 molal C12H22O11(aq) will have the lowest melting point and pure water will have the highest melting point.