### 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?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=134&t=26482

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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?

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?

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.

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.