Question

How many moles is 135 L of ammonia gas at STP?

Answer 1

`6.02 "moles"`

Explanation:

Since we're at STP, we can use the ideal gas law equation.
`PxxV = nxxRxxT`.

• 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 `(Lxxatm)/ (molxxK)`)
• T represents the temperature, which must be in Kelvins.

Next, list your known and unknown variables:

`color(red)("Known variables:")`
`P`
`V`
`R`
`T`

`color(blue)("Unknown variables:")`
`n`

At STP, the temperature is 273K and the pressure is 1 atm. The proportionality constant, R, is equal to 0.0821 `(Lxxatm)/ (molxxK)`

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

`n = (PV)/(RT)`

Plug in the given values:

`n = (1cancel"atm"xx135cancelL)/(0.0821(cancel"Lxxatm")/(molxxcancel"K")xx273cancel"K"`
`n = 6.02 mol`