# Help with partial gas law problem

## Homework Statement

A 32.0 mL sample of hydrogen is collected over water at 20°C and 750.0 torr pressure. What is the volume of the dry gas at STP? (vapor pressure of water at 20°C = 17.5 torr)

## Homework Equations

Ptot = Pdry + Pwet

P1V1/T1 = P2V2/T2

## The Attempt at a Solution

Pdry = Ptot - Pwet = 750 - 17.5 = 732.5 torr

V1 = P2V2T1/P1T2 = (732.5 torr)(32 mL)(298K)/(760 torr)(293 K) = 31.3 mL

DrClaude
Mentor
What is STP?

STP is standard temperature and pressure -- which I used
T = 25.0 °C ( 298 Kelvin) and
P = 1 atm of pressure (760 torr)

Did I use these in the correct variables? Perhaps that is where I am confused?

DrClaude
Mentor
STP is standard temperature and pressure -- which I used
T = 25.0 °C ( 298 Kelvin) and
P = 1 atm of pressure (760 torr)
That's SATP. STP is T = 0°C.

Ah, what a silly mistake. It works out now!

Thanks so much DrClaude! I appreciate your time.

DrClaude
Mentor
Ah, what a silly mistake. It works out now!
You would be surprised how commen this mistake is. This is probably what SteamKing was alluding to. the main problem is that most thermodynamic is for 298 K, but STP was defined as 273 K.

Thanks so much DrClaude! I appreciate your time.
You're welcome!