dissolution of NaOH, amount of energy released

Question

When we add 1g of NaOH(s) into 50ml of water. The temperature rises by 4 degree celsius. Calculate the amount of energy relased in this reaction.

In this case, I understand that we use

Q=mc deltaT to calculate. What I don't understand is why we are using 50g as mass instead of 51g (1g of NaOh and 50ml of water) even though the final temperature we mesured is the one of water plus NaOH (aq) dessolved.

Answer 1

In more advanced chemistry courses, they do teach to add mass of solute, but the rationale for not adding it is that as soon as the NaOH begins to dissolve, heat is absorbed by the 50 ml (50 g) of water and not by 51 g of solution. As more and more NaOH dissolves, then the mass would approach 51 g but for most of the dissolution process, the mass is less than 51 g and more than 50 g.

q = (50 g)(4.184 J/g/deg)(4 deg) = 836.8 joules = 830 joules (to 2 significant figures)