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Converting J/kg to kJ/mol

by clintyip
Tags: converting, j or kg, kj or mol
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clintyip
#1
Aug18-11, 06:33 AM
P: 22
Let's say I have a value of:

553 x 10^5 j/kg

How would I convert that to kJ/mol?
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rl.bhat
#2
Aug18-11, 07:14 AM
HW Helper
P: 4,433
mol = mass/molecular weight. = kg/M
kJ = 10^3 J
clintyip
#3
Aug18-11, 07:17 AM
P: 22
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...)

Tomer
#4
Aug18-11, 07:18 AM
P: 202
Converting J/kg to kJ/mol

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.
clintyip
#5
Aug18-11, 07:19 AM
P: 22
Quote Quote by Tomer View Post
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 kg 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.
Well, this is actually a fuel, Kerosene, with a molecular weight of 120...
Tomer
#6
Aug18-11, 07:23 AM
P: 202
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.
clintyip
#7
Aug18-11, 07:27 AM
P: 22
Quote Quote by Tomer View Post
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.
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
Tomer
#8
Aug18-11, 07:34 AM
P: 202
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.
clintyip
#9
Aug18-11, 07:44 AM
P: 22
Quote Quote by Tomer View Post
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.
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?
Tomer
#10
Aug18-11, 07:47 AM
P: 202
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!
clintyip
#11
Aug18-11, 07:49 AM
P: 22
Haha, thanks! :)
rl.bhat
#12
Aug19-11, 04:10 AM
HW Helper
P: 4,433
Quote Quote by clintyip View Post
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?
1 kg of kerosene produces 55300 kJ of energy.
How much energy is produced by 0.120 kg? That is required energy per mol.


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