How do u do this one?

Question

A gas sample with a mass of 2.50 g is collected at 20.0ºC and
732.5 mm Hg. The volume is 1.28 L. What is the molar mass of the gas?

Answer 1

You would use the ideal gas law PV = nRT

If you are working with L and mm Hg, the gas constant R is 62.3637.

n is the number of moles and T is the temperature in Kelvin.

n = PV / RT
= 732.5 x 1.28 / ( 62.3637 x 293.15)
= 0.05129 moles

Molar mass = 2.5g / 0.05128 moles = 48.7g

Answer 2

You can also solve it this way:

Use: P1V1/T1 = P2 V2/T2; solve for V2 to get liters at STP;

P1V1T2/T1P2 = [732.5 x 1.28 x 273]/[293 x 760] = 1.149mol

Now determine determine molar mass; remember 1 mole of any gas is 22.4 liters at STP!

2.5g/[1.149liters/22.4l/mol] = 48.7g/mol

Answer 3

You need to use the ideal gas law - PV = nRT and solve for n (number of moles). Once you have moles, you can solve for molar mass (MM) as MM = grams (2.50g) / moles (you calculated above as n).

You could do this all in one step substituing m/MM for n in the ideal gas law, but if you are just starting, I think its best to break it into two steps.