How to calculate the enthalpy change from the reaction of silver nitrate and sodium iodide? When $\pu{20.00 mL}$ of $\pu{1.00 M}$ $\ce{AgNO3}$ solution is added to $\pu{20.00 mL}$ of $\pu{1.00 M}$ of $\ce{NaI}$ at $\pu{25 ^\circ C}$ in a calorimeter, a white precipitate of $\ce{AgI}$ is formed. The temperature of the aqueous mixture increases to $\pu{40 ^\circ C}$.


I am trying to calculate the $\Delta H$ for the reaction per mole of $\ce{AgI}$.
The specific heat of the aqueous mixture is $\pu{4.184 J//g K}$, the density of the mixture $\rho = \pu{1.00 g//ml}$. And I assume that the calorimeter absorbs a negligible amount of heat. 

How am I suppose to go about finding the $\Delta H$ per mole of $\ce{AgI}$? 
I don't really know where to start here as I am unclear about the whole problem here. In order to use the formula 
$$Q = m C_p \Delta T$$
I need to find the mass. But which mass do I take? $\ce{AgI}$, or $\ce{NaNO3}$,  or is it the mass of the entire product of the reaction? 

This is the balanced equation
$$\ce{AgNO3 + NaI -&gt; AgI + NaNO3}$$
When the reaction takes place and releases its heat, everything in the calorimeter  heats up: the water, the $\ce{AgI}$ and the $\ce{NaNO3}$. But of these the water is by far the most massive and for that reason, as a reasonable approximation, we consider that mass that absorbs the heat to be the water, i.e. $40.00\ \mathrm{g}$.

Use this to calculate $Q$.

Then determine how many moles of $\ce{AgI}$  was formed, by stoichiometry. The quotient of these two numbers gives you the reaction enthalpy per mole of $\ce{AgI}$