Average Molecular weight question.........?

Question

A mixture of methane (CH4) and ethane (C2H6) contains 30 wt% ethane. The average molecular weight of this mixture is?

Answer 1

Molar mass methane = 16g

Molar mass ethane = 30g

Mixture is 70% methane and 30% ethane

16*70= 1120

30*30 = 900

Total = 2020

Average molar mass = 2020/100 = 20.20g

Answer 2

Dr. A 's answer is correct.  I do agree with his answer.

Answer 3

We have 30 g of ethane => MM = 30.07 g/mol and 70 g of methane => MM = 16.043 g/mol

Moles ehane = 0.998

Moles methane = 4.36

total moles =5.36

Total mass = 100 g

average molecular weight = 100 g / 5.36 =18.7 g/mol