Hey there !
Given the reaction:
N2 + 3 H2 = 2 NH3
At constant pressure and temperature ,volume is proporcional to moles:
Theoretical moles of N2 and H2 => 1:3
Theoretical volume of N2 and H2 => 1:3
Experimental volume of N2 and H2 => 3.0 L : 4.0 L
0.75 : 1 = 2.25 : 3
Since N2 is in excess reactant , H2 is the limiting reactant
Therefore:
volume of NH3 is 2/3 * Volume of H2
= 2/3 * 4.0 = 2.66 L
Hope that helps!