Aevenir V.

asked • 01/04/20# A gas has a density of 0.791 g/L. What is the molar mass of this gas?

I wonder why the formula is m (mass) = p x V (density multiply with volume)

But the answer key of the book is: Molar Mass (M) = g/L × 22.4 L/1 mole = p x V

SO , in this situation , do we find mass = molar mass= p x V ????

## 1 Expert Answer

J.R. S. answered • 01/04/20

Ph.D. in Biochemistry--University Professor--Chemistry Tutor

Assuming STP conditions and 22.4 L = 1 mole of gas, you can relate the following:

density = g/L x 22.4 L/mole = g/mole

0.791 g/L x 22.4 L/mole = 17.72 g/mole = **17.7 g/mole** (3 sig. figs.)

Doing it the long way and using the ideal gas law: PV = nRT and assume standard temperature and pressure (since these variables aren't given in the question), then you have ...

P = 1 atm

V = 1 L

n = ? moles

R = 0.0821 Latm/Kmol

T = 273K

Solving for n (moles), we have...

n = PV/RT = (1 atm)(1 L)/(0.0821 Latm/Kmol)(273K)

n = 0.04462 moles

Since the 1 L contains 0.791 g, we now have...

0.791 g/0.04462 moles = 17.727 g/mole = **17.7 g/mole** (to 3 significant figures)

The answer key of p x V is actually ρ x V where ρ is density or mass/volume, so you have

mass/volume x volume = mass (not molar mass). Not really sure what your question is regarding the ρ and V, but hopefully I've explained this sufficiently. If you have further questions, post them in the comments sections.

Aevenir V.

Hi Mr.J.R.S, Thank you so much for your explanation. It is really explicit and meaningful for me. However, I have not studied about the ideal gas law PV = nRT so it is a bit confused for me to understand it. Do you have any simple way that do not need to apply the ideal gas law ? Anyways, I feel so thankful and a lot of thanks, sir !!!01/04/20

J.R. S.

01/04/20

Barry M.

01/05/20

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Aevenir V.

A gas has a density of 0.791 g/L. What is the molar mass of this gas? (3 points) Selected Answer: Molar mass (gram/moles) = g/L x 22.4 = 0.791 x 22.4 ≈ 17.72 (gram/moles)01/04/20