87.4 Activity and activity coefficient 1. Concepts H=uH(T)+RTinayt Hcr=f- (t)+rTinaci HCI HCI CI=FHCI+RT in(qH.ac) WHCI=AHCI (T)+rT In aHcI Therefore: a Because solution only containing single ion does not exist, the activity of individual ion is unmeasurable, therefore, we use mean activity instead mean activity a=a,a=at a,Va=(a,a)HCl HCl HCl   = + ( ) ln T RT a Therefore: HCl H Cl a a a =  + − Because solution only containing single ion does not exist, the activity of individual ion is unmeasurable, therefore, we use mean activity instead. mean activity 2 + − =  a = a a a 2 1 ( ) a = a = a+ a− H H H   + + + = + ( ) ln T RT a Cl Cl Cl   − − − = + ( ) ln T RT a HCl H Cl    = ++ − HCl H Cl    = ++ − HCl HCl H Cl   = +  RT a a ln( ) + − 1. Concepts §7.4 Activity and activity coefficient