# A substance has a molar mass of 40.0g/mol. If the decomposition of 1.00mole of this substance absorbs 60.0KJ of energy, how much energy would be absorbed by the decomposition of 20.0g of the substance?

Nov 5, 2015

$\text{30.0 kJ}$

#### Explanation:

The idea here is that you need to use the given molar mass to determine how many moles you'd get in the $\text{20.0 g}$ sample.

So, a substance's molar mass tells you what the exact mass of one mole of said substance is. In your case, the substance has a molar mass of $\text{40.0 g/mol}$, which means that every mole will have a mass of $\text{40.0 g}$.

If that's the case, then you can say that

20.0color(red)(cancel(color(black)("g"))) * "1 mole"/(40.0color(red)(cancel(color(black)("g")))) = "0.500 moles"

So, if $\text{60.0 kJ}$ of heat are being absorbed when $1.00$ moles undergo decomposition, it follows that the decomposition of $0.500$ moles wil require

0.500color(red)(cancel(color(black)("moles"))) * "60.0 kJ"/(1.00color(red)(cancel(color(black)("mole")))) = color(green)("30.0 kJ")