Question

a sample of ethane ( C₂H₆) gas has the same mass as 1.5 x 10²⁰ molecules of methane (CH_​4). how many C₂H₆ molecules does the sample of gas contain?

Answer 1

No. of molecules methane = 1.5 $\times$10²⁰ Molecules
Mass of one mole of C₂H₆​ =30 g
Mass of one mole of CH₄​ =16 g
​1 mole of substance contains 6.022$\times$10²³ molecules
Mass of 6.022$\times$10²³ molecules of Methane = 16 g
​Mass of 1.5$\times$10²⁰ molecules of methane =16$\times$1.5$\times$10²⁰ / 6.022$\times$10²³
= 3.98$\times$10^-3
30 g of Ethane contains 6.022$\times$10²³ molecules
3.98$\times$10^-3 g of ethane will contain 6.022$\times$10²³ $\times$ 3.98$\times$10^-3​ / 30
= 0.8$\times$10²⁰ molecules
= 8$\times$10¹⁹ molecules