Question

If 30.6 calories are required to vaporize 25g of water, what is the heat of vaporization (in cal/mole) of that substance?

Answer 1

$25g$ of water contains

$$\nu=\dfrac{25g}{18g/mole} = \dfrac{25}{18}mole$$

mole of substance. Thus, the heat of vaporization is the following:

$$h = \dfrac{30.6cal}{25/18mole} \approx 22cal/mole$$

Answer.  22 cal/mole.