*A 0.465 g sample of an unknown compound occupies 245 mL at 298 K and 1.22 atm. What is the molar mass of the unknown compound?*
A. 38.0 g/mol

B. 56.0 g/mol

C. 12.2 g/mol

D. 33.9 g/mol

E. 81.8 g/mol

**Solution:** Molar mass has units of grams/moles. Therefore, if we have grams and divide them by moles, we will get molar mass. Grams are given and we need to find moles. We are given:

V = 245 mL = 0.245L

T = 298 K

P = 1.22 atm

We can use the Ideal Gas Law to find the number of moles:

PV=nRT

1.22 atm*0.245L = n (0.0821 Latm/molK)(298 K)

n = 0.0122 moles

Now, lets find the molar mass.

Molar mass = 0.465 g/0.0122moles = 38.0 Choice A