## Homework Problem 8.37

$\Delta U=q+w$

Saloni Patel 1H
Posts: 15
Joined: Fri Sep 25, 2015 3:00 am

### Homework Problem 8.37

I do not understand why in this problem the equation for the enthalpy of vaporization is not used, but instead $\Delta h= \Delta q / n$ is used? should it not be the intial equation we learned that is $\Delta h _v_a_p = H _v_a_p- H _l_i_q_u_i_d$?

504650118
Posts: 11
Joined: Fri Sep 25, 2015 3:00 am

### Re: Homework Problem 8.37

We use the first equation you mention because that's the only information given in the problem. You can also find the enthalpy of vaporization by the second equation but we are not given the enthalpy values for the liquid and vapor. We do know that enthalpy of vaporization is the heat required to vaporize the liquid, so we just take the amount of heat needed and divide it by the number of mols to find the energy needed to vaporize a mol of the substance. (Part B gives the substance in grams, so you must convert that to mols).

Estefania Lima 2G
Posts: 14
Joined: Fri Sep 25, 2015 3:00 am

### Re: Homework Problem 8.37

We cannot use $\Delta H_{vap}=H_{vap}-H_{liquid}$ because we are not given those values. The problem gives us a ratio of kJ to moles, (which are the units for enthalpy) so we can use that ratio to find $\Delta H$. You can look at it like a proportion:
$\frac{4.67 kJ}{0.579 mol}=\frac{x kJ}{1 mol}$ What you get when you solve for x is the value of the enthalpy of vaporization.