Calculate the molarity of a solution of ethanol in water in which the mole fraction of ethanol is 0.040 (assume the density of water to be one).

Mole fraction = 0.04/1 = 1/25.

In 25 moles of the total mixture, 1 mole of ethanol and 24 moles of water are present.

24 moles of water weigh 24 x 18 = 432 g = 432 mL (18g is the molar mass of water)

1 mole of ethanol is present in 432 mL of water.

Molarity is no. of moles of solute present in 1 L of solution.

So molarity = 1 x 1000 / 432

= 1000/432

= 2.315 M (approx.)