CHEM 108 1st Edition Lecture 20Chapter 16 Acid–Base and Solubility Equilibrium continuedWeak Acids:- HNO2 (aq) + H2O(l) ⇌ H3O+ (aq) + NO2 − (aq)- To calculate pH of:o strong acid; [H+] = [Acid]initial o Weak acid; [H+] = calculated by solving equilibrium equation using I.C.E table. Once we have the equilibrium concentrations we can find the pH because we have[H+] concentration.o Same procedure for weak base.Weak Bases:- Add weak base to water: B + H2O BH+ + OH-o Weak base: equilibrium lies to the left (Kb small)- Obtain [OH−] at equilibrium using I.C.E table, then use Kw relationship.Ionization:- Degree of ionization: o (quantity of substance that is ionized)/concentration of substance before ionizationo If degree of ionization is small it is a weak acid or baseo The closer it is to 1 the stronger the acid or base- Percent ionization: degree of ionization expressed as a percent. o This can be used to validate an approximation when determining if the K value is too small or not If greater than 5% then approximation is not valid If less than 5% then approximation is valido HA(aq) A-(aq) + H+ (aq) Ka = ([A-][H+ ])/([HA]) Percent ionization = [H+]equilibrium/ [HA]intial* 100%- % ionization decreases as [Acid]0 increases. Polyprotic acid:- Polyprotic acid: can donate more than one proton- Examples: H2SO4, H2CO3 , H3PO4 - If Ka for dissociation steps differ by three orders of magnitude, only need to use first dissociation step to calculate pH - Diprotic acids: two ionizable H atoms. - Triproticacids: three ionizable H atoms (Phosphoric acid, Citric acid)- There is a different value of Ka for each ionizable level o Ka1> Ka2 (> Ka3 …) - More difficult to remove H+ ion (positive charge) from negatively charged anion. - pH of Polyprotic acid solutions: Typically only first ionizable H atom affects pH
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