Question

What is the mole fraction of solute and the molal concentration for an aqueous solution that is 16.0% NaOH by mass?

Answer 1

let's start:

16 g NaOH has 84 g of water 
moles of NaOH =16/40
                          =0.4 
moles of water =84/18
                    = 4.667 
Total moles = 0.4+4.667
                   = 5.067 
mole fraction of NaOH = 0.4/5.067
                                     = 0.0789
Conclusion: 
84 g water will have 0.4 moles of NaOH 
1000g water will have [0.4x 1000]/84
                                  = 4.76 moles
This is called molality.
ENJOY :)