Answer : The theoretical yield of water in this reaction is 360 g.
Solution : Given,
Mass of 
Mass of 
Mass of 
Molar mass of
= 44 g/mole
Molar mass of
= 24 g/mole
Molar mass of
= 18 g/mole
First we have to calculate the moles of
and
.
Moles of
= 
Moles of
= 
The given balanced chemical reaction is,

From the given reaction, we conclude that
1 mole of
react with 2 mole of 
20 moles of
requires
of 
But the actual moles of
= 42 moles
Excess moles of
= 42 - 40 = 2 moles
So,
is the limiting reagent.
Now we have to calculate the mass of
.
From the reaction, we conclude that
1 mole of
gives 1 mole of 
20 moles of
gives 20 moles of 
Mass of
= Moles of
× Molar mass of 
Mass of
= 20 moles × 18 g/mole = 360 g
The experimental yield of
= 325 g
Therefore, the theoretical yield of
= 360 g