Question

How many moles are in a gas in 890mL at 21 °C and 750 mmHg?

Answer 1

`n="0.036 mol"`

Explanation:

Use the ideal gas law. The formula is `PV=nRT`, where `P` is pressure, `V` is volume, `n` is moles, `R` is the gas constant, and `T` is the Kelvin temperature.

Given/Known
`P="750 mmHg"="750 Torr"`
`V=890cancel"mL"xx(1"L")/(1000cancel"mL")="0.89 L"`
`R="62.363577 L Torr K"^(-1) "mol"^(-1)`
https://en.m.wikipedia.org/wiki/Gas_constant
`T="21"^@"C"+273.15"=294 K"`

Unknown
`n`

Solution
Rearrange the formula to isolate `n`. Substitute the given/known values into the formula and solve.

`PV=nRT`

`n=(PV)/(RT)`

`n=(750cancel"Torr" * 0.89cancel"L")/((62.363577 cancel"L" cancel"Torr" cancel"K"^(-1)" mol"^(-1))xx(294cancel"K"))="0.036 mol"` rounded to two significant figures