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Topic: Thermochemistry - estimate mass of water evaporate...  (Read 17653 times)

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Offline viet

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Thermochemistry - estimate mass of water evaporate...
« on: October 14, 2008, 02:01:16 PM »
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Evaporating sweat cools the body because evaporation is an endothermic process: H2O (l) => H2O (g)  /\Hrxn = +44.01 kJ

Estimate the mass of water that must evaporate from the skin to cool the body by 0.50C. Assume a body mass of 85 kg and assume that the specific heat capacity of the body is 4.0 J/g*C.

m = ____ g

heres what i have:
q = m * Cs * /\T
q = 85000 * 4.0 J/gC * 0.50C
q = 170,000 J

im not sure what to do next, is there a way to solve this problem without the heat of vaporization formula? havent learn that yet.

Offline Astrokel

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Re: Thermochemistry - estimate mass of water evaporate...
« Reply #1 on: October 14, 2008, 04:26:13 PM »
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q = 170,000 J

If you required this amount of energy to evaporate and that per mole of evaporation reaction is +44.01KJ, how many moles of water required, thus its mass?
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Offline viet

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Re: Thermochemistry - estimate mass of water evaporate...
« Reply #2 on: October 14, 2008, 05:37:21 PM »
so i divide 170,000 J by 44.01 kJ:  170000/44.01 = 3862.76 K. how do i get it in to grams?

Offline Astrokel

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Re: Thermochemistry - estimate mass of water evaporate...
« Reply #3 on: October 15, 2008, 12:29:06 AM »
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170000/44.01 = 3862.76

check your unit

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how do i get it in to grams?
mole ---> mass
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline viet

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Re: Thermochemistry - estimate mass of water evaporate...
« Reply #4 on: October 15, 2008, 05:29:32 PM »
is this right?

170000/ 44010 = 3.9
3.9 (18.01g H2O/1mol) = 69.6g

Offline Astrokel

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Re: Thermochemistry - estimate mass of water evaporate...
« Reply #5 on: October 16, 2008, 12:25:54 AM »
Yes, correct, maybe you want to use more sig fig(3-4) in your intermediate working.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

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