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I want to calculate the energy change when a solute is dissolved in water. I know that I can achieve this by using the equation $q=mc \Delta T$. My question is does the mass ($m$) change with the amount of solute added to the solvent?
For example, if I had 25g of water ($c=4.18$) as my solvent and 5g of sodium chloride as my solute. Would the energy change equal:
$$q=25 \cdot 4.18 \cdot \Delta T$$ or $$q=(25+5) \cdot 4.18 \cdot \Delta T$$
Your energy conservation equation is $$q_{rxn} + q_{soln} = 0$$ so $$q_{rxn} = -q_{soln} = -m_{soln}c_{soln}\Delta T_{soln}$$ Note the minus sign; if $\Delta T$ is negative, the energy will be positive, and vice versa. And yes, you need the mass of the solution here, and the specific heat capacity of the solution too.
If you only need a couple of significant figures and the solution isn't too concentrated, you can assume $c_{soln} \approx c_{\rm H_2O}$.
If you need more precision, though, you'll need to measure or look up $c_{soln}$. And you'll need to include the heat absorbed by the calorimeter in your energy conservation equation, too.
You have to take the mass of the mixture. So, the second expression is true.
