7,8 г                                                            Х л

 2K        +         2H2O       =       2KOH     +   H2

n = 2 моль                                                  n = 1 моль

Mr = 39 г/моль                                           Vm=22,4 л/моль

m = 78 г                                                      V = 22,4 л

7,8 г К     -     Х л Н2

78 г К      -     22,4 л Н2

V(H2) = 7,8 * 22,4 / 78 =2,24 л