Experimental values, At $STP.$

$V_{1}=40 \,\mathrm{mL}$

$V_{2}=?$

$T_{1}=300 \,\mathrm{K}$

$T_{2}=273 \,\mathrm{K}$

$P_{1}^{2}=725-25=700\, \mathrm{mm}$

$P_{2}=760\, \mathrm{mm}$

$\frac{P_{1} V_{1}}{T_{1}}=\frac{P_{2} V_{2}}{T_{2}}$

$V_{2}=\frac{P_{1} V_{1} T_{2}}{T_{1} P_{2}}=\frac{700 \times 40 \times 273}{300 \times 760}=33.52\, \mathrm{mL}$

$22400\, \mathrm{mL}$ of $\mathrm{N}_{2}$ at $STP$ weighs $=28\, \mathrm{g}$

$\therefore 33.52\, \mathrm{mL}$ of $\mathrm{N}_{2}$ at $STP$ weighs $=\frac{28 \times 33.52}{22400}$

$=0.0419\; \mathrm{g}$

$\%$ of $\mathrm{N}=\frac{\text { Mass of nitrogen at STP }}{\text { Mass of organic compound taken }} \times 100$

$=\frac{0.0419}{0.25} \times 100=16.76 \%$