Mass of solution or solvent in calculating ΔHsol? (1 Viewer)

[sodiumbromate]

E.g. Calculate the molar heat of solution when 1.837g of NaOH dissolves in 150mL of water, if ΔT=4.32°C.
ΔHsol=-mCΔT/n(solute)=-(151.837x4.18x4.32)/0.0459... or
-(150x4.18x4.32)/0.0459...

I've been told to use both by different teachers, which is kinda distressing. Which one do we use for HSC questions?

 

[ml125]

Use the 150g mass. This is because you're initially using the temperature change and specific heat capacity of water, so any additions made to the mass of water would imply the addition of more water.

 

[sodiumbromate]

Use the 150g mass. This is because you're initially using the temperature change and specific heat capacity of water, so any additions made to the mass of water would imply the addition of more water.

Ohhh ok thanks for the clarification. It seems kinda obvious now but I was genuinely conflicted by sources saying things like "each subsequent molecule or ion is dissolving into a mixture of dissolved solute in solvent. In this case, the final solute molecule or ion is dissolving into a solution with a mass approximated by the mass of the solvent plus the mass of the solute" and "it is assumed that c(solution) = c(solvent)"