Question

A 2.00 liter evacuated container has a mass of 1050.0 g. When the container is filled with an unknown gas at 800. mm Hg pressure and 25.0 °C the mass is 1052.4 g. What is the molar mass of the gas (in g-mol"')?
(A) 28
(B) 31
(C) 54
(D) 56

Answer 1

Answer:

(A) 28

Explanation:

To solve this problem we use the PV=nRT equation, where:

• P = 800 mmHg ⇒ 800/760 = 1.05 atm

• V = 2.00 L

• n = ?

• R = 0.082 atm·L·mol⁻¹·K⁻¹

• T = 25.0 °C ⇒ 25.0 + 273.16 = 298.16 K

We input the data:

• 1.05 atm * 2.00 L = n * 0.082 atm·L·mol⁻¹·K⁻¹ * 298.16 K

And solve for n:

• n = 0.086 mol

Now we calculate the gas' mass:

• Gas Mass = (Mass of Container w/ Gas) - (Mass of Empty Container)

• Gas Mass = 1052.4 g - 1050.0 g = 2.4 g

Finally we calculate the unknown gas' molar mass, using its mass and its number of moles:

• Molar Mass = mass / moles

• Molar Mass = 2.4 g / 0.086 mol = 27.9 g/mol

So the answer is option (A).