# Enthalpy of Combustion

#### Integral0

Enthalpyof Combustion of 1 molar volume of H2(g) divided by Enthalpyof Combustion of 1 molar volume of CH4(g)

=

Enthalpyof Combustion per mole of H2 divided by Enthalpy of Combustion per mole of CH4

why???

#### Bystander

Homework Helper
Gold Member
Why does x/y = mx/(my)?

#### Integral0

Re

I guess . . . I'm just writing down what the book says . . .

#### Integral0

RE

I am asking why is one molar volume equal to one mole in this case at hand?

#### Monique

Staff Emeritus
Gold Member
Re: RE

Originally posted by Integral0
I am asking why is one molar volume equal to one mole in this case at hand?
It isn't :)

Where does it say that the enthalpy of combustion for one mole or one molar is the same? The only thing the same, is the answer of the division, the fraction (by lack of a better word).

#### Integral0

RE

why are the two expressions above (my first post) the same or equal?

My previous comment was wrong . . . I was just trying to piece together why they would be the same. "I am asking why is one molar volume equal to one mole in this case at hand?"

#### Bystander

Homework Helper
Gold Member
Please paraphrase your question --- the only thing that is clear is that you are struggling with a definition --- which definition it is, is not clear.

#### Monique

Staff Emeritus
Gold Member
OK, lets look at it in the following way. Hypothetical numbers.

The enthalpy of burning 2 liter one molar H2 is x.
The enthalpy of burning 2 liter one molar CH4 is 2x.

So H2/CH4 would be x/2x.

Now, lets look at moles.

The enthalpy of burning one mole H2 is 0.5x
The enthalpy of burning one mole CH4 is x

So H2/CH4 would be 0.5x/x, which can rewritten as x/2x.

So x/2x = 0.5x/x.

#### Integral0

RE

but the first expression says "Molar volume" while the second one says "per mole"?

Are they the same?

What is there relation to eachother?

#### Bystander

Homework Helper
Gold Member
Paraphrasing, "Does 'molar' mean 'per mole'?" In this instance, yes. "Molar volume" does mean volume per mole, and one molar volume is a way of saying one mole. Likewise, "molar enthalpy" is the same thing as saying enthalpy per mole.

For an instance in which "molar" does not imply one mole, consider 10 ml of a one "molar" solution --- "x molar" means x moles per liter of solution at whatever temperature is indicated for the glassware.

#### Monique

Staff Emeritus
Gold Member
Well, molar volume just means moles/liter, it is a concentration. While per mole is an absolute amount.

.. a few cogs have turned in my head .. maybe what is meant with a 1 molar volume, is the volume of gas that defines the amount of 1 mole??

hgm..

#### Integral0

RE

well, if "per mole" and molar volume can be used interchangeable in this situation, why did they use both?

I think we aren't understanding each other here.

I am asking in the question why can the two expressions equal each other since one specifies "Enthalpy of Combustion Per MOLE" and the other specifies "Enthalpy of Combustion of 1 molar volume".

Is the "Enthalpy of Combustion Per MOLE" the same as the "Enthalpy of Combustion of 1 molar volume"?

Molar volume is the volume occupied by one mole of ideal gas at STP. Its value is:

22.414 L mol¯1

Enthalpy of Combustion is the change of enthalpy in the standard state of formation per mole.

Any insights MONIQUE?

#### Monique

Staff Emeritus
Gold Member
AH! SEE! Good that you kept asking Integral0 :)

Here is the definition: The molar volume is the volume occupied by one mole of ideal gas at STP.

The unit is L mol^-1

So no matter what, you are always dealing with one mole, if you are talking about 1 molar volume, or per mole.

#### Integral0

awesome!

PM me about biochemistry (your major concentration in college) . . .

Do you like it? etc . . . Experiences?

#### Monique

Staff Emeritus
Gold Member
Re: RE

hehe, sorry about talking past each other.. it is a strange example..

Originally posted by Integral0
well, if "per mole" and molar volume can be used interchangeable in this situation, why did they use both?
To illustrate that they can be used interchangeably?

Is the "Enthalpy of Combustion Per MOLE" the same as the "Enthalpy of Combustion of 1 molar volume"?
yes

#### Monique

Staff Emeritus
Gold Member
Re: awesome!

Originally posted by Integral0
why, thanks. It had been a while that I had used the expression molar volume.. thanks for reminding me about it

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