# chem question can't figure it out

#### teefRcool

##### Senior Member
10+ Year Member
When 1.00g of liquid water (MW = 18) is reduced from H2 and O2 at a constant temperature (25C) and pressure (1 atm) 15.8 kilojoules are produced. What is the molar heat of formation of liquid water, in kilojoules?

can someone please show how they do this i'm soo lost with this one.

thanks

#### allstardentist

##### All-Star
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isnt it just, -15kj/(1*stiochimetric coeff. of H2O/18) ?
not sure tho

#### Immuno-guy

##### Member
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I'm not sure. Perhaps multiply by 18 to get the value for one mole (18g/mol for h2o).

Any thoughts?

10+ Year Member
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-(15.8 kj * 18)

#### Clapton

##### Member
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2H2O -> 2H2 + O2 = 15kJ

-15 kJ x 2molH2O = -30kJ

They try to trick you with the MW of water. You don't need it.

#### aranjuez

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Clapton said:
2H2O -> 2H2 + O2 = 15kJ

-15 kJ x 2molH2O = -30kJ

They try to trick you with the MW of water. You don't need it.

Clapton's right, but if I remember correctly, you don't multiply by 2mol, rather, you divide by it. -15kJ is the kJ for the formation, which is what we want to look at.

2H2 + O2 -> 2H2O = -15kJ

But this is the heat of formation of 2 moles of H20. Therefore, the heat of formation for 1 mol is just half of that. Answer is -15/2kJ, or -7.5kJ.

Do you have the answer, by any chance?

aranjuez

#### Clapton

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aranjuez said:
Clapton's right, but if I remember correctly, you don't multiply by 2mol, rather, you divide by it. -15kJ is the kJ for the formation, which is what we want to look at.

2H2 + O2 -> 2H2O = -15kJ

But this is the heat of formation of 2 moles of H20. Therefore, the heat of formation for 1 mol is just half of that. Answer is -15/2kJ, or -7.5kJ.

Do you have the answer, by any chance?

aranjuez

No, I'm sure you multiply. The heat of formation is given for 1 mole of water. In the balanced equation, there are 2 moles. They give you H2 and 02. For every mole of O2, there has to be 2 moles of H2 in order to make water. Two waters have to be formed from what they give you.

#### aranjuez

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Clapton said:
No, I'm sure you multiply. The heat of formation is given for 1 mole of water. In the balanced equation, there are 2 moles. They give you H2 and 02. For every mole of O2, there has to be 2 moles of H2 in order to make water. Two waters have to be formed from what they give you.

The heat of formation is given for the reaction, which you have to balance. You can't assume 1mol H20. I could be wrong, but that's how I see it.

aranjuez

#### Clapton

##### Member
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My bad. I forgot that it said 1g. The heat of formation is for 1 gram of water, not 1 mole. In this case, you just multiply -15.8 x 18 I think.

#### cryptozoologist

##### Junior Member
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reverse reaction gets you water as a product...so heat of formation changes signs.

15.8 kj / g x 18 g / mol

mental check: you have to REMOVE heat to turn gases into a liquid.