The atomic mass of potassium is 19 g/mol 90.0 g of potassium contains ; = 90.0/19 = 4.737 moles The reaction between potassium and nitrogen is given by the equation; 6K + N2 = 2K3N The mole ratio of potassium and nitrogen is 6:1 The number of moles of nitrogen; = 4.737/6 = 0.7895 moles But, 1 mole of nitrogen occupies 22.4 liters at STP Therefore; 0.7895 moles × 22.4 liters = 17.685 liters