# How many moles of He (g) are in a 5 L storage tank filled with He at 10.5 atm pressure and 30°C?

Dec 28, 2016

Approx. $2 \cdot m o l$

#### Explanation:

$n = \frac{P V}{R T} = \frac{10.5 \cdot a t m \times 5 \cdot L}{0.0821 \cdot L \cdot a t m \cdot {K}^{-} 1 \cdot m o {l}^{-} 1 \times 303 \cdot K}$

=??mol

The only difficulty with these sorts of problems is selecting the right gas constant $R$.

Chemists tend to use $R = 0.0821 \cdot L \cdot a t m \cdot {K}^{-} 1 \cdot m o {l}^{-} 1$, because the units of $\text{litres}$ and $\text{atmospheres}$ are convenient. Note that you really don't have to remember them because they should be provided to you in an exam.