Molar mass= xCalculating Molar Mass of an Ideal Gas

[UWMpanther]

[SOLVED] Solving for molar mass

Homework Statement

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?

Homework Equations

PV = nRT

The Attempt at a Solution

So n= no. moles present
So I assumed n=.622/x

so : x= .622gRT/PV

 

[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:

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

 

[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?

 

[rocomath]

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?

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.

 

[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

 

[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.

 

[UWMpanther]

Thank you very much!