# Converting J/kg to kJ/mol

1. Aug 18, 2011

### clintyip

Let's say I have a value of:

553 x 10^5 j/kg

How would I convert that to kJ/mol?

2. Aug 18, 2011

### rl.bhat

mol = mass/molecular weight. = kg/M
kJ = 10^3 J

3. Aug 18, 2011

### clintyip

so that means if I had 55300000 j/kg, that would be 55300 kJ/kg.

How would I make it into kJ/mol....

(sorry, i'm not very good at converting...)

4. Aug 18, 2011

### Tomer

from J to kJ all you need to do is of course to devide in 1000 (=k).

It's simply to look at it like this:

10^3j = kj => j = 10^-3kj
so 553 * 10^5 j = 553 * 10^5 * 10^-3 kj = 553 * 10 ^2 kj
(I substituted j for kj using the upper formula).

About kg=>mol, well, for that you'll have to tell us what's the material involved.
Imagine there are plastic and metal balls. A plastic ball weighs 1 kg and a metal ball 2kg.
It'd be like asking you "how many balls are in a sack weighing 10kg?". If you knew it's plastic balls, you'd say 10. If it were metal balls, you'd say 5.

so 1kg can be 1000 mol and it can be 1 mol => we need more information.

5. Aug 18, 2011

### clintyip

Well, this is actually a fuel, Kerosene, with a molecular weight of 120...

6. Aug 18, 2011

### Tomer

Then use the relation given by rl.bhat and derive the connection between kg and mol. When you say 120, make sure if it's 120 g/mol or 120 kg/mol, and convert fittingly.

7. Aug 18, 2011

### clintyip

So I have 55300 kJ/kg

Assuming that my Molecular weight is 120g/mol.....

Yeah, I have no idea. It seems like there's a missing variable to me :S

8. Aug 18, 2011

### Tomer

How come?

rl.baht wrote: mol = mass / molecular weight.

we want to know the connection between kg and mol. So assume you have 1kg material.
Step 1: convert your molecular weight from 120g/mol => X kg/mol (you do it :-)
Step 2: use rl.baht connection written above, instead of mass you have "kg", and instead of molecular weight you have what you've calculated on Step 1.
Step 3: from the formula of Step 2 isolate kg. You'll get something like kg = Y mol.
Step 4: substitute kg for mol in the expression you've got.

Give it a try, it's important and basic. If you won't make it I'll show you the way.

9. Aug 18, 2011

### clintyip

Step One: 120g/mol => 0.12kg/mol
Step Two: mol = mass/molecular weight
= kg/0.12 = mol
kg = 0.12mol

So 55300kJ/0.12mol
= 460833 kJ/mol

Wow, that's a big number, did I do it right?

10. Aug 18, 2011

### Tomer

hehe, the distance between the earth and the sun in centimeters is also a big number and it's still true :-)
About the numbers - I really don't know about the 120 g/kg, you should make sure of that, but it seems you did it perfectly, great!

11. Aug 18, 2011

### clintyip

Haha, thanks! :)

12. Aug 19, 2011

### rl.bhat

1 kg of kerosene produces 55300 kJ of energy.
How much energy is produced by 0.120 kg? That is required energy per mol.