Question

A 425 mL volume of hydrogen chloride gas, `HCl`, is collected at 25°C and 720 torr. What volume will it occupy at STP?

Answer 1

The volume at STP will be 370 mL, rounded to two significant figures.

Explanation:

Since your question involves the pressure, volume, and temperature of HCl gas, you will need to use the combined gas law in order to solve this problem. Temperature will need to be converted to Kelvins and pressure will need to be converted to kPa.


The current value for STP as determined by the IUPAC (International union of Pure and Applied Chemistry), is `"273.15 K"` and `10^5 "Pa"`, which is usually written as `"100 kPa"` because it is easier to work with.

Known
`P_1="720 torr"xx(0.133322"kPa")/(1"torr")="96 kPa"`
`V_1="425 mL"`
`T_1="25"^@"C+273.15=298 K"`
`P_2="100 kPa"`
`T_2="273.15 K"`

Unknown
`V_2`

Rearrange the equation in order to isolate `V_2`. Substitute in the known values and solve.

`V_2=(P_1V_1T_2)/(T_1P_2)`

`V_2=(96"kPa"xx425"mL"xx273.15"K")/(298"K"xx100"kPa")="370 mL"` rounded to two significant figures