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### 9.19

Posted: Sun Jan 28, 2018 5:04 pm
What equation do you use for the equation and why? What given values or key phrases in a word problem do you look for when determining to use this equation?

### Re: 9.19

Posted: Sun Jan 28, 2018 5:06 pm
I have similar question because you use the equation deltaS= Cp(ln(T2/T1), but I am confused as to if we are supposed to include the moles/grams in the equation or ignore them all together?

### Re: 9.19

Posted: Sun Jan 28, 2018 9:31 pm
I think you have to first know what changes are happening in this process. There are 3 steps to this process:
1. Reversible heating of water liquid from 85 degree C to 100 degree C
For this you use the molar heat capacity at constant pressure of liquid water
ΔS=n.Cp,m.ln(T2/T1)
2. Phase change from liquid to vapor
For this you use the standard entropy of vaporization at 100 degree C
ΔS=(ΔSvap).n
3.Water vapor cools from 100 degree C to 85 degree C
For this you use the the molar heat capacity at constant pressure of water vapor
ΔS=n.Cp,m.ln(T2/T1)

And yes, you have to include the moles in the equation because the numbers given are molar heat capacities and standard entropy whose units are J/K/mol so you have to multiply them with number of moles to get the entropy change. For this exercise, I just assume nH2O=1.00 mol.