If air contains 78%N2 by volume ,what will be solubility of nitrogen im water at 25⁰c

C = pK

p = 0.78 atm since it is 78% by volume in air and air pressure is 1 atm.
Look up K for N2 @ 25 C in mols/L*atm. CAUTION. K is often given in other units too.
Solve for C in moles/L. If you want to convert to grams/L then grams = mols/L x molar mass = ?
Post your work if you get stuck.