Mole fraction?

Question

What is the mole fraction of NaOH in an aqueous solution that contains 15% NaOH (weight/weight%)?

Answer 1

the mole fraction is (moles of solute) / (moles of solution)

 

Since the solution is 15% NaOH by weight, the easiest approach is to assign some numbers for calculating:

 

NaOH = 15 g

solution = 100 g

water = 85 g

 

Because we are calculating a ratio, the actual weights involved are irrelevant. So we determine the moles of NaOH and the moles of water based on the assigned weights.

 

NaOH = 15 g / 40 g/mole = .375 moles NaOH

 

H₂O = 85 g / 18 g/mole = 4.72 moles H₂O

 

total = .375 + 4.72 = 5.1 moles solution

 

mole fraction = .375 / 5.10 = .074