A103.8g sample of nitric acid solution that is 70.0% hno3 contains by mass

w/w percentage 
               = mass of the pure compound / total mass of the sample x 100%

70% HNO₃ contains by mass means every 100 g of sample has 70 g of HNO₃.

The mass of solution = 103.8 g
Hence the mass of HNO₃ = 103.8 g x 70%
                                         = 103.8 g x (70 / 100)
                                         = 72.66 g = 72.7 g.

The mass has escaped into a gas which you can not see so the law stays true.