Homework Help: Unit Conversions: kJ/mol to kJ/litre

1. Aug 24, 2011

clintyip

Let's say I have 4163 kJ/mol of energy coming from a substance with molar mass 86.14g/mol. How would I convert the 4163 kJ/mol of energy to kJ/litre?

Thanks.

2. Aug 24, 2011

Staff: Mentor

You'll need to also know the density (g/L) of the substance.

3. Aug 24, 2011

clintyip

The density of this substance is 0.6548 g/mL.

4. Aug 24, 2011

lightgrav

You multiply by a number that has moles on top , so they cancel.
but now there's something else on the bottom,
so you multiply by a number that has grams on top . then you're done.

5. Aug 25, 2011

clintyip

But I don't have a number with mols on top o_o

6. Aug 25, 2011

Staff: Mentor

Conversion factor

$$\frac {86.14 g} {1 mol}$$

can be reversed to

$$\frac {1 mol} {86.14 g}$$

The idea is that conversion factor equals 1, switching nominator and denominator doesn't change the value, although it moves units.

7. Aug 25, 2011

clintyip

That means, I can multiply 4163kj/mol by 1/86.14g, which then I get 4163kJ/86.14g.
Then I multiply this number by the density. So 4163kJ/86.14g x 0.6578g/mL
which equals to 31.64 kJ/mL

Is that right?

8. Aug 25, 2011

Staff: Mentor

To be precise by 1 mol/86.14g, but I guess it is just a typo.

OK so far, but you are not there yet - you need to convert it to kJ/L. But that's the simplest part.

9. Aug 25, 2011

clintyip

Don't I just multiply it by 1000? So I get 31645.4 kJ/L.

But the thing is, this substance is Hexane, and according to Wikipedia, it has a heat of combustion of 29.3 kJ/L. The two numbers aren't even remotely close.

10. Aug 25, 2011

Staff: Mentor

Yes.

Obviously what they mean is MJ/L, otherwise their numbers are not consistent. As much as I like wikipedia, it is hardly a definite source.