The molar mass of nh3 is 17.03 g/mol. the molar mass of h2 is 2.0158 g/mol. in a particular reaction, 0.575 g of nh3 forms. what is the mass, in grams, of h2 that must have reacted, to the correct number of significant figures?

The answer 
the general for of the equation of reaction is 
N2        +     3 H2   2NH3
1mol            3mole                2 mole
                    nH2                   nNH3
(nX means number of mole of X)

according to the law of proportion
3 / nH2  =  2 / nNH3  (E)

nNH3 = m NH3  / 17.03 , and m NH3 = 0.575 g
consequently nNH3 =  0.575 g  / 17.03 = 0.03 mol

by considering (E)  3 / nH2  =  2 / 0.03 = 66.66, and nH2  = 3 / 66.66 =0.045

finally we can write 

mH2 = nH2 x 2.0158 =0.045 x 2.0158 = 0.09 g