Assume ideal gas behavior. How many moles of vapor are present?

PV = nRT

P = pressure in atm
V = volume in L
n = moles of gas
R = ideal gas constant (0.0821 L atm / mol K)
T = temperature in Kelvin (K)

The problem tells us:

P = 754 torr ~ 1 atm
V = 152mL = .152L.... When it tells us that we have  125mL Erlenmeyer, this is unnecessary information. The problem says we have a MEASURED volume of 152mL. So this is what the gas occupies, and therefore this is what affects the properties of the gas under ideal conditions.

R = .0821 Latm/molK (just a known number. memorize this)
T = 98.7 C = 371.7 K

We can solve for moles now:

(1)(.152) = n (.0821)(371.7)

n = .005 moles of vapor present


Given in the problem: 0.199 grams of vapor

So we calculate the number of moles, and now knowing the mass we have, we can calculate the molar mass.

Moles = grams / molecular mass

.005 = .199 / Molar mass

Molar mass= 0.199 / .005

Molar mass = 39.95

Hope that helped!

This helped me a lot!