Sel Test 7.9A How do we calculate H(vapor)?

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Jennifer Aguayo 1I
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Joined: Fri Sep 26, 2014 2:02 pm

Sel Test 7.9A How do we calculate H(vapor)?

Postby Jennifer Aguayo 1I » Wed Jan 21, 2015 6:00 pm

A sample of benzene, C6H6, was heated to 80 degrees Celsius, its normal boiling point. The heating was continued until 15.4 kJ had been supplied; as a result, 39.1g of boiling benzene was vaporized. What is the enthalpy of vaporization of benzene at its boiling point?
For this problem I understand that we use the equation:
Delta H= H(vapor) - H(liquid)
But I am not sure how to start it, can someone help me please.

Thank you!

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

Re: Sel Test 7.9A How do we calculate H(vapor)?

Postby Niharika Reddy 1D » Wed Jan 21, 2015 8:40 pm

I'm not entirely sure if this is how you do it, but this is how I approached the problem and got the same answer as the textbook.

ΔHvap is the energy as heat required to vaporize a mole of benzene (units: kJ/mol).
We are given the amount of energy required to vaporize 39.1 g of benzene (15.4 kJ). Converting the mass of benzene to moles using the molar mass of benzene (78.11 g/mol) gives 0.500 mol benzene. Dividing 15.4 kJ by the amount (moles) of benzene that that much heat vaporized gives the heat of vaporization of benzene in kJ/mol:

ΔHvap = 15.4 kJ / 0.500 mol = 30.8 kJ/mol

Jennifer Aguayo 1I
Posts: 26
Joined: Fri Sep 26, 2014 2:02 pm

Re: Sel Test 7.9A How do we calculate H(vapor)?

Postby Jennifer Aguayo 1I » Wed Jan 21, 2015 11:18 pm

Okay that makes sense.
So for this type of question we do not have to do use Delta H=Hm(vapor)- Hm(Liquid)? How would we know when to use this equation?

Justin Le 2I
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Joined: Fri Sep 26, 2014 2:02 pm

Re: Sel Test 7.9A How do we calculate H(vapor)?

Postby Justin Le 2I » Sat Jan 24, 2015 8:20 pm

For this equation:
Delta H= H(vapor) - H(liquid)
Are you talking about the delta H of the entire heating process?

Jennifer Aguayo 1I
Posts: 26
Joined: Fri Sep 26, 2014 2:02 pm

Re: Sel Test 7.9A How do we calculate H(vapor)?

Postby Jennifer Aguayo 1I » Thu Jan 29, 2015 10:22 pm

Hi Justin,
Sorry for the late reply. No I was asking how to solve Delta Hvap .


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