# How many moles of oxygen are in a 5.5-L canister at STP?

Jun 24, 2016

There are $0.25 m o l$ of ${O}_{2}$ in the 5.5-L canister.

#### Explanation:

We are at STP, so we have to use the ideal gas law equation.
$P \times V = n \times R \times T$.

• P represents pressure (could have units of atm, depending on the units of the universal gas constant)
• V represents volume (must have units of liters)
• n represents the number of moles
• R is the universal gas constant (has units of $\frac{L \times a t m}{m o l \times K}$)
• T represents the temperature, which must be in Kelvins.

Next, list your known and unknown variables. Our only unknown is the number of moles of ${O}_{2} \left(g\right)$. Our known variables are P,V,R, and T.

At STP, the temperature is 273K and the pressure is 1 atm. The proportionality constant, R, is equal to 0.0821 $\frac{L \times a t m}{m o l \times K}$

Now we have to rearrange the equation to solve for n:

$n = \frac{P V}{R T}$

n = (1cancel"atm"xx5.5cancelL)/(0.0821(cancel"Lxxatm")/(molxxcancel"K")xx273cancel"K"
$n = 0.25 m o l$