Question

5. Calculate the molarity as well as the mass percentage of a 10.0% v/v aqueous solution of ethanol. The density of pure ethanol is 0.789 g/ml. The density of the aqueous solution is 0.985 g/ml.

Answer 1

10 v/v % ethanol means 10 mlethanol in 100 ml water

So volume of ethanol = 10 ml
density of ethanol= 0.789 g/ml

thus mass of ethanol = volume*density = 10 ml*0.789 g/ml
=7.89 g

volume of solution = 100ml = 0.1 L
density of solution = 0.985 g/ml

mass of solution = 100 ml*0.985 g/ml = 985 g

now we need number of moles of solute for the calculation of molarity
number of moles of ethanol= given mass of ethanol/molar mass of ethanol
= 7.89 g/46.07 g/mol = 0.17126 mole

molarity = number of moles of ethanol/volume of solution in L
= 0.17126 moles/0.1 L = 1.7126 M

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Mass percent =[ mass of solute/mass of solution]*100

= 7.89 g/985 g*100 = =0.8 %
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