A measure of the tendency of a gas to escape or expand. Fugacity () is the pressure value needed at a given temperature to make the properties of a non-ideal gas satisfy the equation for an ideal gas, i.e.,
where is the fugacity coefficient and is the partial pressure for component i of the gas. For an ideal gas,
) is the pressure value needed at a given temperature to make the properties of a non-ideal gas satisfy the equation for an ideal gas, i.e., 
is the fugacity coefficient and 
is the partial pressure for component i of the gas. For an ideal gas,