# Homework Help: Solving for molar mass

1. Dec 15, 2007

### UWMpanther

[SOLVED] Solving for molar mass

1. The problem statement, all variables and given/known data
What is the molar mass of an ideal gas if a 0.622g sample of this gas occupies a volume of 300mL at 35*C and 789mm Hg?

2. Relevant equations
PV = nRT

3. The attempt at a solution
So n= no. moles present
So I assumed n=.622/x

so : x= .622gRT/PV

2. Dec 15, 2007

### rocomath

$$n=\frac{grams}{\frac{grams}{mole}}=\frac{grams}{M}$$

Solve for big M.

Make sure you convert all your units, R has the units:

$$R=\frac{L*atm}{K*mol}$$

Last edited: Dec 15, 2007
3. Dec 15, 2007

### UWMpanther

Ok so I am very confused here. How do I know how many moles I have? And do I need the entire PV= nRT formula?

4. Dec 15, 2007

### rocomath

You do not need to know how many moles there are. I accidentally mistyped it, hopefully you see that I fixed it.

Eventually, you will end up with the units g/mol, and you'll be fine from there.

And yes, you need the entire equation, because R will allow you to attain the units moles and it will cancel out the other ones you do not need.

5. Dec 15, 2007

### UWMpanther

is R not a constant? 0.08206 $$L\ atm\ mol^-^1\ K^-^1$$

so $$M=\frac {grams\ast R\ast T} {P\ast V}$$

convert 3mL to .300L and 789mm Hg to 1.038atm

6. Dec 15, 2007

### rocomath

No R is a constant, I'm not sure what I said to imply it wasn't but it is.

You're answer is correct, you get g/mol once you cancel out common units.

7. Dec 15, 2007

### UWMpanther

Thank you very much!!!!