Question

What is the mole fraction of NH₃ in a 10.0 m aqueous solution of NH₃?

(1) 0.02          (2) 0.15             (3) 0.18             (4) 0.82              (5) 0.85

Answer 1

Consider 1 Kg of solvent H2O

m(solvent)= 1 Kg

use:

number of mol,

n = Molality * mass of solvent in Kg

= (10 mol/Kg)*(1 Kg)

= 10 mol

Now calculate mol of H2O

Molar mass of H2O,

MM = 2*MM(H) + 1*MM(O)

= 2*1.008 + 1*16.0

= 18.016 g/mol

mass(H2O)= 1 Kg

= 1000 g

use:

number of mol of H2O,

n = mass of H2O/molar mass of H2O

=(1*10^3 g)/(18.02 g/mol)

= 55.51 mol

Now use:

Mole fraction of NH3 = mol of NH3 / total number of moles

= 10 / (10 + 55.51)

= 0.15

Answer: option 2