[BreakingBad20]

(a)      An average man weighs about 70kg and produces about 10 460 kJ of heat per day. His heat capacity is
 4.18 J K-1 g -1
(ii) A man is in fact an open system, and the main mechanism for maintaining his temperature constant is evaporation of water.  If the enthalpy of vaporization of water at 37^oC  is 43.4 kJ mol -1, how much water needs to be evaporated per day to keep his temperature constant?

I got part (i) which is irrelevant to this question. Here is my answer but I don't think it's right:
E = (M_man)(C)(deltaT) + (M_water)(L)
10460000 = (70)(4.18)(37) + M(43400)
M = 240.76kg

[enahs]

Your vaporization enthalphy is in units of mols, not mass or liters. Also, your heat capacity for a man is in units of grams, but you used in your calculations kg.

Also, how does a man have a  :delta: T of 37? What is the units of your heat capacity? Does it have T in it?


Units!



Perhaps you should post the whole question, as part I might not be irrelevant and it might help.

[BreakingBad20]

Q.1   
(a)      An average man weighs about 70kg and produces about 10 460 kJ of heat per day.

(i) Suppose that a man is an isolated system and that his heat capacity is
 4.18 J K-1 g -1; if his temperature were 37C at a given time, what would his temperature be 24h later?

(ii) A man is in fact an open system, and the main mechanism for maintaining his temperature constant is evaporation of water.  If the enthalpy of vaporization of water at 37C  is 43.4 kJ mol -1, how much water needs to be evaporated per day to keep his temperature constant?    

So should I leave delta T to be zero because the temperature has to be constant leaving the equation to be used E = ML for the water?

[enahs]

I re-updated my post after you posted, go back and check it real quick. Units!

[BreakingBad20]

I understand what you mean about the units but my new problem is that if the temperature of the man is to be kept constant does that not mean delta T (either in kelvin or degrees Celsius) is going to be zero? This would then cancel out (M_man)(C)(deltaT) would it not?

[enahs]

The question says his temperature is constant. So essentially you are assuming there is this steady state of energy flowing in and out of the man, and I believe you can neglect his heat capacity.

[BreakingBad20]

So am I right to assume E = (M_water)(L) for part (ii) ?
if so:
10460 kJ = (M)(43.4 kJ/mol)
M_water = 241.01 mol
> mass in grams = moles x Mr
> mass in grams = 241.01 x 18.016
> mass in grams = 4342.03616g
Mass in Kilograms = 4.342 Kg
This certainly looks a lot more accurate than my first attempt.

[enahs]

It is totaly possible to sweat 4-5 liters a day. I would say that is not average at all, for just body maintenance. Unless you live in a dessert, or workout a lot, etc.  So I feel happy about it I guess.

[BreakingBad20]

ya i suppose ... thanks anyway!