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.)