# Help with partial gas law problem

1. Apr 1, 2013

### gurerd

1. The problem statement, all variables and given/known data
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)

2. Relevant equations

Ptot = Pdry + Pwet

P1V1/T1 = P2V2/T2

3. 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

2. Apr 2, 2013

What is STP?

3. Apr 2, 2013

### SteamKing

Staff Emeritus
4. Apr 2, 2013

5. Apr 2, 2013

### gurerd

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?

6. Apr 2, 2013

### Staff: Mentor

That's SATP. STP is T = 0°C.

7. Apr 2, 2013

### gurerd

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

Thanks so much DrClaude! I appreciate your time.

8. Apr 3, 2013

### Staff: Mentor

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.

You're welcome!