Five liters of gas at STP have a mass of 12.5 g. What is the molecular mass of the gas?

a. 12.5 g/mol

b. 25.0 g/mol

c. 47.5 g/mol

d. 56.0 g/mol

e. 125 g/mol

You have 12.5 grams. And 5 liters of some random gas.

Step 1: You must find the mol amount of that 12.5g mass.

12.5 / 5 = 2.5 mol

Step 2: Now find the molecular mass of that gas "at STP" (STP - is 22.4 L)

2.5 x 22.4 = 56 g/mol

Good Luck on Chemistry SAT, cause I know exactly where you got this question from.