Question

# A gas mixture is made by combining 7.4 g each of Ar, Ne, and an unknown...

A gas mixture is made by combining 7.4 g each of Ar, Ne, and an unknown diatomic gas. At STP, the mixture occupies a volume of 18.28 L.

What is the molar mass of the unknown gas? =    g/mol

Given

Pressure = 1 atm ( at STP)

Temperature = 273.15 (at STP)

Volume = 18.28 L

Now consider, PV = nRT

1 x 18.28 = n x 0.0821 x 273.15

n =   18.28 / 22.4

= 0.816 mole

Number of moles of Argon = Mass / Molar mass

= 7.4 / 39.95

= 0.185mol

Number of moles of Ne = mass / molar mass

= 7.4 / 20.18

= 0.3666 mol

Number of moles of unknown gas = 0.816 -0.185 - 0.3666

= 0.26 mol

The molar mass of an unknown gas is = mass / Moles of unknown

= 7.4 / 0.26

= 28.4 g / mol

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