From this reaction, n2(g) + 2h2(g) > 2nh3(g)
if 20 grams of products are formed, how many grams of reactants did you start with?

The answer to your question is:

16.47 g of N2 and 3.52 g of H2

Explanation:

Data  

20 g of products were obtain.

Process

AW N2 = 28 g

AW H2 = 2g

MW NH3 = 17 g

                                     N2(g) + 3H2(g) ---> 2NH3(g)

                                     28g       3(2)g            2(17) g

                                                                        20 g

N2

                28 g  N2  34 g of NH3

                 x              20 g of NH3

x = 20(28)/34 = 16.47 g of N2

H2            6 g of H2  34 g of NH3

                  x                                       20 g of NH3

x = 20(6)/34 = 3.52 g of H2

Total mass = 16.47 g of N2 + 3.52 g of H2 = 19.99 g ≈ 20 g