Calculate the density of a gas if the mass is 2.94g, the volume is 756mL at 20C and 500torr, and ....?

Question

the conditions are changed to STP

a. 5.51x10(-3)g/mL
b. 2.38x10(-3)g/mL
c. 6.34x10(-3)g/mL
d. 5.34x10(-3)g/mL

Answer 1

Density is mass / volume,
D = m / v

Since you are given the mass and the volume in the problem, the temperature and pressure are not important (beyond specifying the conditions which the density was measured).

Density = 2.94 grams / 756 mL
Density = .00389 g/mL, @ 500 torr, 20 degrees C

EDIT:
Oh, wait, I see...you wanted to find the new density after the conditions are changed to STP.

Remeber the ideal gas law,
PV = nRT

Answer 2

Since you need to change the conditions to STP, you'll need to do this problem in a couple of steps. First, given the conditions that you have, you can use the Ideal Gas Law to calculate the number of moles of the gas present under these conditions. Second, you'll use that number of moles to calculate the volume of the same gas at STP. Finally, you can divide the mass you're give initially by the final volume to determine the density of the gas at STP.