[kalyanvadagam]

 we can calculate Heat released when NH4Cl dissolved in water with the formula
Q=mcΔt.

m= Mass of overall mixture after dissolution
C= specific heat of water
Δt= temp change

why we take mass of both NH4Cl and water together and why we take only specific heat of water.

why we dont include NH4Cl specific heat figure if we use its mass for calculation of heat change

[Borek]

For sufficiently diluted solutions their specific heat is for most practical purposes close enough to that of water.

You are right that exact calculations require a bit different approach. Using specific heat of a solid NH₄Cl won't work though, what you need is the exact specific heat of the solution.