Editing Thermodynamic activity (section)

===Activity coefficient===
{{Main|Activity coefficient}}
The activity coefficient {{mvar|γ}}, which is also a dimensionless quantity, relates the activity to a measured [[mole fraction]] {{mvar|x<sub>i</sub>}} (or {{mvar|y<sub>i</sub>}} in the gas phase), [[molality]] {{mvar|b<sub>i</sub>}}, [[mass fraction (chemistry)|mass fraction]] {{mvar|w<sub>i</sub>}},  [[molar concentration]] (molarity) {{mvar|c<sub>i</sub>}} or [[mass concentration (chemistry)|mass concentration]] {{mvar|ρ<sub>i</sub>}}:<ref name="McQuarrie&Simon"> McQuarrie, D. A.; Simon, J. D. ''Physical Chemistry – A Molecular Approach'', p. 990 & p. 1015 (Table 25.1), University Science Books, 1997. </ref> 
:<math>a_{ix} = \gamma_{x,i} x_i,\ a_{ib} = \gamma_{b,i}\frac {b_i} {b^{\ominus}},\, a_{iw}=\gamma_{w,i} w_i,\ a_{ic} = \gamma_{c,i} \frac{c_i}{c^{\ominus}},\, a_{ir} = \gamma_{\rho,i} \frac{\rho_i}{\rho^{\ominus}}\, </math>

The division by the standard molality {{math|''b''<sup><s>o</s></sup>}} (usually 1 mol/kg) or the standard molar concentration {{math|''c''<sup><s>o</s></sup>}} (usually 1 mol/L) is necessary to ensure that both the activity and the activity coefficient are dimensionless, as is conventional.<ref name="GreenBook"/>

The activity depends on the chosen standard state and composition scale;<ref name="McQuarrie&Simon"/> for instance, in the dilute limit it approaches the mole fraction, mass fraction, or numerical value of molarity, all of which are different. However, the activity coefficients are similar.{{citation needed|date=February 2022}}

When the activity coefficient is close to 1, the substance shows almost ideal behaviour according to [[Henry's law]] (but not necessarily in the sense of an [[ideal solution]]). In these cases, the activity can be substituted with the appropriate dimensionless measure of composition {{mvar|x<sub>i</sub>}}, {{math|{{sfrac|''b<sub>i</sub>''|''b''<sup><s>o</s></sup>}}}} or {{math|{{sfrac|''c<sub>i</sub>''|''c''<sup><s>o</s></sup>}}}}. It is also possible to define an activity coefficient in terms of [[Raoult's law]]: the [[International Union of Pure and Applied Chemistry]] (IUPAC) recommends the symbol {{mvar|f}} for this activity coefficient,<ref name="GreenBook"/> although this should not be confused with [[fugacity]].
:<math>a_i = f_i x_i\,</math>