Standard Temperature and Pressure: MDCAT Chemistry notes
Standard Temperature and Pressure for MDCAT: STP values, units of pressure (atm, torr, mm Hg, Pa, psi), temperature scales and molar volume 22.4 dm³.
What is STP?
Standard temperature and pressure (STP) are the reference conditions used to compare gas volumes:
- Standard temperature: 0 °C = 273.15 K (often taken as 273 K) = 32 °F.
- Standard pressure: 1 atm = 760 torr = 760 mm Hg = 76 cm Hg = 101325 Pa = 14.7 psi.
Do not confuse STP with room conditions (25 °C, 298 K).
Units of pressure
Pressure is force per unit area. Its SI unit is the pascal: $1\ \text{Pa}=1\ \text{N m}^{-2}$.
| Unit | Value of 1 atm |
|---|---|
| Atmosphere | 1 atm |
| Millimetres of mercury | 760 mm Hg (= 76 cm Hg) |
| Torr | 760 torr (1 torr = 1 mm Hg) |
| Pascal | 101325 Pa (= 101.325 kPa) |
| Pounds per square inch | 14.7 psi |
| Millibar | 1013.25 mbar (1 bar = $10^5$ Pa) |
Worked examples
- Half an atmosphere: $\frac{101325}{2}=50662$ Pa, which is also 380 torr and about 7.35 psi.
- Convert 380 mm Hg to atm: $\frac{380}{760}=0.5$ atm.
- Convert 2 atm to kPa: $2\times101.325=202.65$ kPa.
Temperature scales
$$T(\text{K})=t(^\circ\text{C})+273.15\qquad ^\circ\text{F}=\frac{9}{5}(^\circ\text{C})+32$$
So 0 °C = 273 K = 32 °F, and 100 °C = 373 K = 212 °F. Gas-law calculations must always use kelvin.
Molar volume
By Avogadro's law, one mole of any ideal gas occupies 22.414 dm³ (about 22.4 dm³) at STP and contains $6.02\times10^{23}$ molecules. This allows direct volume-to-mole conversions:
$$n=\frac{V\ (\text{dm}^3)}{22.4}$$
Example. In $2H_2+O_2\rightarrow2H_2O$, 1 mol of water needs 1 mol $H_2$, which is 22.4 dm³ at STP. Making 1 mol water needs 0.5 mol $O_2$, i.e. 11.2 dm³.
Example. 44.8 dm³ of $CO_2$ at STP is 2 mol, i.e. 88 g.
Common MDCAT traps
- 1 atm is 14.7 psi, not 1.47 psi.
- Standard temperature is 0 °C, not 25 °C and not 0 K.
- The SI unit of pressure is the pascal; torr, bar and mm Hg are not SI units.
- Standard temperature on the Fahrenheit scale is 32 °F, not 0 °F.
- Molar volume 22.4 dm³ applies only to gases, not to water.
Quick revision
- STP: 273 K and 1 atm.
- 1 atm = 760 torr = 101325 Pa.
- 1 torr = 1 mm Hg.
- Molar volume at STP = 22.4 dm³.
- 0 °C = 32 °F.