Answer
See the answer below.
Work Step by Step
From Henry's Law: $S=k_HP=2.4\times10^{-2}\ mol/kg.bar\times1.00\ bar=2.4\times10^{-2}\ mol/kg$
Mass of water: $500.\ mL\times 1.00\ g/mL=500\ g$
N. of moles of N2O: $2.4\times10^{-2}\ mol/kg\times 500\ g\div 1000\ g/kg=0.012\ mol$
Mass of N2O: $0.012\ mol\times 44.01\ g/mol=0.528\ g$
ppm concentration: $0.528/(500+0,528)\times10^6\ ppm=1055\ ppm$