Gas mixtures

Question

A gas mixture is made by combining 7.2g each of Ar, Ne, and an unknown diatomic gas. At STP, the mixture occupies a volume of 17.07L. What is the molar mass of the unknown gas in g/mol, and identify the unknown gas?

Answer 1

At STP, the mixture occupies a volume of 17.07L

So, 17.07 L  x (1 mole/22.4L) = .7621 moles

7.2g Ar x (1 mole/39.948 g) = .1802 mole Ar

7.2g Ne x (1 mole/20.1797 g) = .3568 mole Ne

Add them: .5370 moles of Ar Ne mixture

.7621 moles - .5370 moles = .2251 moles of unknown gas

7.2 g/ .2251 mol = 131.99 g/ mol

That's pretty close to Zenon.