# Change in internal energy for the heating and vaporisation of water

What is the value of change in internal energy at $\pu{1atm}$ in the process $$\ce{H2O(l,323\ \mathrm K)->H2O(g,423\ \mathrm K)}$$ given $C_{\mathrm m,V}(\ce{H2O,l})=\pu{75.0 J K^{-1} mol^{-1}}$ and $C_{\mathrm m,p}(\ce{H2O,g})=\pu{33.314 J K^{-1} mol^{-1}}$

a. $\pu{42.91 kJ/mol}$
b. $\pu{43086 kJ/mol}$
c. $\pu{42.6 kJ/mol}$
d. $\pu{49.6 kJ/mol}$

I tried to use the First Law here. For chemical reaction I found $\Delta n_\mathrm g$ to be $1$ and the heat of the reaction $\Delta H$ is given as $\pu{40.7 kJ mol-1}$. However, I am not able to use the specific heat data given in the question. What am I doing wrong here?

I am getting stuck in phase change reactions, particularly in using $C_p$ and $C_V$ here. I know the First Law will work, but how do we use $C_p$ and $C_V$ in such reactions?

• @GENESECT I got you are started by replacing the image. That should help to format the other parts of the post. Also, option d was cut off in your image.
– Tyberius
Commented Aug 17, 2018 at 17:12
• It seems like you might have to use the density of water and the ideal gas law to approximate the change in volume as water goes from the liquid to vapor state. Commented Aug 17, 2018 at 19:14