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Mole fraction?

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

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