Molar mass= xCalculating Molar Mass of an Ideal Gas

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To calculate the molar mass of an ideal gas, the formula M = (grams * R * T) / (P * V) is used, where R is the ideal gas constant. In this case, a 0.622g sample occupies 300mL at 35°C and 789mm Hg. It is essential to convert the volume to liters and the pressure to atmospheres for accurate calculations. The number of moles does not need to be known directly, as the equation will yield the molar mass in g/mol. The discussion confirms that using the entire PV=nRT equation is necessary for proper unit cancellation and obtaining the final result.
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[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
 
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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:
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?
 
UWMpanther said:
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.
 
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
 
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.
 
Thank you very much!
 
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