The molar mass of argon is 40g / mol what is the molar mass of a gas if it effuses at 0.91 times the speed of argon gas

To determine the molar mass of the unknown gas, we use Graham's Law of Effusion where it relates the effusion rates of two gases with their molar masses. It is expressed as r1/r2 = √M2/M1. We calculate as follows:

Let 1 = argon gas
      2 = unknown gas

r2 = 0.91r1
r1/r2 = 1/0.91

1/0.91 = √M2/M1 = √M2/40
M2 = 48.30 g/mol

Copper, silver, aluminum, cobalt, chromium, mercury 

16- counting by fours