Answer : The mass of hydrogen gas consumed will be, 4.5 grams
Explanation : Given,
Mass of
= 21 g
Mass of
= 18 g
Molar mass of
= 28 g/mole
Molar mass of
= 2 g/mole
First we have to calculate the moles of
and
.


Now we have to calculate the limiting and excess reagent.
The balanced chemical reaction is,

From the given balanced reaction, we conclude that
As, 1 mole of
react with 3 moles of 
So, 0.75 moles of
react with
moles of 
From this we conclude that,
is an excess reagent because the given moles are greater than the required moles and
is a limiting reagent because it limits the formation of product.
The moles of hydrogen gas consumed = 2.25 mole
Now we have to calculate the mass of hydrogen gas consumed.


Therefore, the mass of hydrogen gas consumed will be, 4.5 grams