Question

Why are 1 molar and 1 molal solutions of glucose different concentrations?

Answer 1

Its because molality and molarity have different definitions.

Explanation:

A 1 mol/kg solution of glucose contains 1 mol sucrose in 1 kg water.

A 1 mol/L solution of glucose contains 1 mol sucrose in 1 L of solution.

The density of the solution is 1.066 g/mL.

∴ The mass of 1 L solution is

`1000 color(red)(cancel(color(black)("mL solution"))) × "1.066 g solution"/(1 color(red)(cancel(color(black)("mL solution")))) = "1066 g solution"`

Thus the solution contains 1 mol glucose (180.16 g) in 0.886 kg water.

The two solutions each contain 1 mol glucose but different amounts of water.