3 moles of hydrogen occupy a volume of 67.20 L, at standard temperature and pressure (STP, 0 ˚C and 760 mmHg). What is the density of hydrogen at STP?

The correct answer is 0.089 g/L ≅ 0.09 g/L

Explanation:

Density is defined as: mass/volume.

From the problem we have:

number of moles = n = 3 mol

volume = V = 67.20 L

We have to calculate the mass. For this, we need the molar mass (MM) of the gas. That is easily calculated from the molar mass of the element hydrogen (H), as we know that hydrogen gas has the molecular formula H₂:

MM(H₂) = 2 x molar mass H = 2 x 1 g/mol = 2 g/mol

Now, we multiply n by MM to obtain the mass (m) of the gas:

m = n x MM(H₂) = 3 mol x 2 g/mol = 6 g

Finally, we calculate the density from the mass and volume:

density = m/V = 6 g/(67.20) = 0.089 g/L ≅ 0.09 g/L