Question

Calculate the mass of the following gas sample at STP. 1.32 L SF6

Answer 1

Step 1 - Remembering the molar volume of a gas

Every gas, at STP, occupies the same volume. We call it the molar volume because it is usually given in the unities volume per mol. So, the volume of a gas at STP will depend only on the quantitie and not on the nature of the gas. The molar volume is 22.4 L/mol, at STP.

Step 2 - Finding the number of moles of SF6

Since we already know the volume occupied by 1 mole, we can set the following proportion to discover how many moles are contained in 1.32 L of SF6:

$\begin{gathered} 1\text{ mole-----22.4 L} \\ x\text{ moles ---- 1.32 L} \\ \\ x=\frac{1.32}{22.4}=5.9\times10^{-2}\text{ moles} \end{gathered}$

Step 3 - Converting moles to mass

Now that we have obtained the number of moles (n) contained in 1.32 L, we need to multiply it by the molar mass (M) of the gas in order to obtain the mass (m). In this case, since the gas is SF6, the molar mass is 146 g/mol.

We have thus:

$n=m\mathrm{}M=146\times5.9\times10^{-2}=8.61\text{ g of SF6}$

So, for 1.32 L of SF6 at STP, the mass is 8.61 g.