Question

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).
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Answer 1

Moles of ethanol = 0.04 mol

Moles of water = 1 - 0.04 = 0.96 mol

Mass of ethanol = 0.04 mol x 46 g/mol = 1.84 g

Mass of water = 0.96 mol x 18 g/mol = 17.3 g

Mass of the solution = 17.3 g + 1.84 g = 19.1g

Assuming density of water to be 1 g/ml,

Volume of solution = mass / density = 19.1 / 1 = 19.1 ml = 0.0191 L

Molarity is the number of moles of solute present in 1 L of solution.

Hence,

Molarity of the solution = 0.040 / 0.0191 = 2.1 M.

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