Question

A sample of ethane (C₂H₆) gas has the same mass as 1.5 ×10²⁰ molecules of methane (CH₄). How many C₂H₆ molecules does the sample of gas contain?

Answer 1

Mass of 1 molecule of CH₄ =  16g/ N_A 

Mass of 1.5 ×1020 molecules of methane = ( 1.5 ×10²⁰  × 16 / N_A)  g

Mass of 1 molecule of C₂H₆ =  30 / N_A g

Mass of molecules of C₂H₆ is =  1.5 ×10 ²⁰​×16/ N_A g 

∴ Number of molecules of ethane = (1.5 X 10²⁰ X 16 / N_A) X N_A / 30 = 0.8 X 10²⁰