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.

Atomicity occurs to prevent updates of the database occurring only partially, which can cause greater problems than rejecting the whole series outright.some are monatomic, diatomic, triatomic etc because it shows the number of atoms. for example, monatomic means it contains only one atom.