2Na+2H2O-->2NaOH+H2
at STP The standard temperature is 273 K (0° Celsius) and the standard pressure is 1 atm pressure. At STP, one mole of gas occupies 22.4 L of volume (molar volume).
so number of moles of H2 is 1
look at the mole ratio of sodium to hydrogen its 2:1