40 г                                                  Х г

 2K           +          2H2O        =        2KOH        +        H2

n = 2 моль                                    n = 2 моль

Mr = 39 г/моль                            Mr = 56 г/моль

m = 78 г                                        m = 112 г

40 г К     -      Х г КОН

78 г К     -     112 г КОН

m(KOH) = 40 * 112 / 78 = 57,4 г