From the equation it is evident that 2 moles of Sodium metal produces 1 mole of hydrogen gas.
At STP 1mole of any gas occupies a volume of 22.4 liters.
Therefore, 7.80 gives---(7.80Ă—1)/22.4 Â moles= 0.3482 moles
Since the mole ratio of Sodium to hydrogen is 2:1, then the number of moles of sodium that reacted is given by the following expression.
(0.3482Ă—2)/1 moles which gives 0.6964 moles.
The atomic mass of  sodium is 23 thus the mass of sodium that reacted is given by:
mass=no. of molesĂ—RAM
0.6964Ă—23= 16.02 grams.
