mass of N2 gas = volume x density
                       Â
= 20.0 x 1.25
                        Â
= 25 g
2 NaN3 ---------------------> 2 Na + 3 N2
130.02
g                                       Â
84g
?                                                  Â
25 g
130.02 g NaN3 --------------------> 84 g N2
x g NaN3 ------------------------> 25 g N2
x = 130.02 x 25 / 84
x = 38.7 g
mass of NaN3 = 38.7 g