# A cylinder containers 27.5 L of oxygen gas at a pressure of 2.0 atm and a temperature of 320K. How much gas (in moles) is in the cylinder?

Jul 4, 2016

A bit over $\text{2 moles}$.

#### Explanation:

We use the Ideal Gas Law:

$n = \frac{P V}{R T}$ $=$ $\frac{2.0 \cdot a t m \times 27.5 \cdot L}{0.0821 \cdot L \cdot a t m \cdot {K}^{-} 1 \cdot m o {l}^{-} 1 \times 320 \cdot K}$ $=$ ??" moles"

The choice of gas constant ($R$) is always problematic in theses sorts of problems. Chemists usually deal with atmospheres and litres. In other problems, sometimes you deal with pascals, and ""^@C, and even (in the States) cubic feet and cubic inches, you may have to use different $R$ values. Because, $a t m$ and $L$ are sensible units, most of the time we use $R = 0.0821 \cdot L \cdot a t m \cdot {K}^{-} 1 \cdot m o {l}^{-} 1$.

If you go thru and cancel the units in the expression above, you'll find that we get an answer in $\text{moles}$. This is a good thing because we wanted an answer in $\text{moles}$, so dimensional analysis is an extra way to check our calculation.