## 4F17

Karina Koo 2H
Posts: 49
Joined: Fri Sep 28, 2018 12:24 am

### 4F17

How do you calculate the standard entropy of vaporization of water at 85C, given that its standard entropy of vaporization at 100C is 109.0 J/K/mol and the molar heat capacities at constant pressure of liquid water and water vapor are 75.3 J/K/mol and 33.6 J/K/mol?

For this problem, I used change in S vap= change in H vap/ Tb
then plugged in the values for vaporization at 100C so
109.0 = change in H vap/ 373 K
change in H vap = 40657

then i plugged in this value for vaporization at 85C
change in S vap = 40657 / 358
change in S vap ended up being 113.57 which is slightly higher than the answer given in the student manual. It also appears like they used a different method. Am i allowed to use the method I used? Thank you!

Jane Burgan 1C
Posts: 73
Joined: Fri Sep 28, 2018 12:15 am

### Re: 4F17

Before doing those two steps, you need to heat the water up from 85C to 100C, so you use the q=mcdeltaT formula, where you use the molar heat capacity of liquid water, plug in moles, plug in temperature difference. After this, you add the result to the other two values that you got in your two steps!

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