STP means standard temperature and pressure. The current values for STP are 0^("o")"C" or "273.15 K" and "10"^5 "Pascals" ("Pa"), usually given as "100 kPa" to make it easier to work with. For the gas laws, the Celsius temperature must be converted to Kelvins by adding 273.15 to the Celsius temperature.
This question can be answered using the equation for the combined gas law:
(P_1V_1)/T_1 = (P_2V_2)/T_2,
where:
P_1 and P_2 are the initial and final pressures, V_1 and V_2 are the initial and final volumes, and T_1 and T_2 are the initial and final temperatures in Kelvins.
Known/Given:
P_1 = 3.00 color(red)cancel(color(black)("atm"))xx(101.325"kPa")/(1color(red)cancel(color(black)("atm")))="303.975 kPa"
V_1 = 720.0 "mL"
T_1 = 20.0^("o")"C" + 273.15 = 293.2"K"
P_2 = "100 kPa"
T_2 = 273.15 "K"
Unknown:
V_2
Solution: Rearrange the combined gas law so that V_2 is isolated, then solve for V_2
V_2 =(P_1V_1T_2)/(T_1P_2) = (303.975color(red)cancel(color(black)("kPa"))*720.0 "mL"*273.15 color(red)cancel(color(black)("K")))/(293.2 color(red)cancel(color(black)("K"))*100color(red)cancel(color(black)("kPa"))) = "2039 mL" (rounded to four significant figures)
Note:
If your teacher is still using "1 atm" for standard pressure, substitute "3.00 atm" for "303.975 kPa", and "1 atm" for "100 kPa". The final volume will be "2010 mL" rounded to three significant figures.
