Question

You made a solution by combining 105.5 grams of ethanol (C₂H₅OH) and 449.5 grams of water. The resulting solution has a density of 0.971 g/ml. With this information answer the following questions.

1. What is the mass percent of ethanol of this solution?

2. What is the molarity of this solution?

3. What is the mole fraction of ethanol of this solution?

4. What is the molality of this solution?

Answer 1

(1) Mass percent of ethanol = ( mass of ethanol /(mass of ethanol + mass of water)) x 100

= ( 105.5 / (105.5+449.5)) x 100

= 19.0 %

(2) Total mass of the solution ,m = 105.5 + 449.5 = 555 g

Density of solution , d = 0.971 g/mL

So Volume of the solution , V = mass / density

= 571.6 mL

So Molarity , M = ( mass/Molar mass of ethanol ) x ( 1000 / volume of solution in mL)

= ( 105.5 g / ( 46(g/mol)) x (1000 / 571.6 mL)

= 4.012 M

(3) Number of moles of ethanol , n = mass/molar mass = 105.5 g / ( 46(g/mol))

= 2.29 moles

Number of moles of water , n' = mass/molar mass

= 449.5 g / ( 18 (g/mol))

= 24.97 mol

Mole fraction of ethanol, X = n / ( n + n' )

= 0.084

(4) Molality , m = ( number of moles of ethanol ) x ( 1000 / Mas of solvent in g)

= 2.29 mol x ( 1000 / 449.5 g)

= 5.094 m