Gases

• standard temperature and pressure (STP)
  • until 1982: $T=273.15\mathrm{K},P=1\text{atm}$
  • since 1982: $T=273.15\mathrm{K},P=1\text{bar}$

Volume of gases

• The molar volume of a substance: $V_m=\frac{V}{n}=\frac{M}{\rho }$ where:
  • $M$ is the molar mass of the substance
  • $\rho$ is the density of the substance
  • $V$ is the volume occupied by the substance
  • $n$ is the amount of substance (in $\mathrm{mol}$)
• SI unit: ${\mathrm{m}}^3/mol$, other units: $L/mol$, $mL/mol$
• at STP: $22.4L/mol$

Ideal gas law

(source: commons.wikimedia.org)
ideal gas law (or general gas equation)	$PV=nRT$
Boyle’s law	$P\propto \frac{1}{V}$
Charles’s law (or law of volumes)	$V\propto T$
combined gas law	$PV\propto T$
Avogadro’s law	$V\propto n$
Gay-Lussac’s law	$P\propto T$

• $P$ is the pressure
• $V$ is the volume
• $n$ is the amount of substance of the gas (in $\mathrm{mol}$)
• $T$ is the absolute temperature
• $R$ is the gas constant (also universal, ideal or molar gas constant)
  • $R=8.3145\mathrm{J}\cdot {\mathrm{K}}^{-1}\cdot \mathrm{mo}{\mathrm{l}}^{-1}$.
  • $R=0.0821\mathrm{L}\cdot \mathrm{atm}\cdot {\mathrm{K}}^{-1}\cdot \mathrm{mo}{\mathrm{l}}^{-1}$
  • $R=N_A{k}_B$
  • $R$ must be expressed in units consistent with those chosen for pressure, volume and temperature.
• $\frac{PV}{TN}=k_b$
  • $N$ is the total number of particles (atoms or molecules) in the gas.
  • $k_b$ is the Boltzmann constant

Van der Waals equation

• $p=\frac{RT}{v-b}-\frac{a}{v^2}$
  • $v$ is the molar volume

Dalton’s law

• Dalton’s law (of partial pressures) ${\chi }_i=\frac{n_i}{n}=\frac{p_i}{p}$
  • total pressure $p={\sum }_{i=1}^n{p}_i$
  • partial pressure $p_i$
  • ${\chi }_i=\frac{n_i}{n}$ is the mole fraction

Henry’s law

• Henry’s law $C=kP$
  • $C$: molarity (concentration) of the dissolved gas in the liquid
  • $k$: Henry’s law constant (depends on the gas, solvent, and temperature)
  • $P$: partial pressure of the gas above the liquid

Vapor pressure

• vapor pressure (לחץ אדים)

  • “the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system” (Wikipedia)
  • higher temperature → higher vapor pressure
  • also called: saturation vapor pressure or equilibrium vapor pressure
  • cf. vapor pressure in meteorology

• https://chemistry.stackexchange.com/questions/34710/relationship-between-vapour-pressure-and-saturated-vapour-pressure

• supersaturation (רוויית יתר)

  • “the condition of a vapour having a higher saturation vapour pressure than is normally possible” (Wiktionary)
  • supersaturated vapor (אד רווי-יתר)

Definition in meteorology

• vapor pressure (לחץ אדים)

  • $e=P_{\mathrm{H}{}_2\mathrm{O}}$
  • the partial pressure of gas (usually water vapor) in the atmosphere

• https://en.wikipedia.org/wiki/Vapor_pressure#Meaning_in_meteorology

• https://glossary.ametsoc.org/wiki/equilibrium-vapor-pressure/

• https://glossary.ametsoc.org/wiki/saturation-vapor-pressure/

humidity

• dew point
• absolute humidity
• relative humidity $=\frac{\text{partial pressure of water vapor}}{\text{saturated vapor pressure of water}}$