Buffers

Solutions of Acids or Bases and Their Salts HF + H2O  H3O+ + F- Problem: The equilibrium [H+] in a 1.0 M HF solution is 2.7 x 10-2 M and the % dissociation is 2.7 % Calculate the [H+] and the percent dissociation of HF in a solution containing 1.0 M HF (Ka = 7.2 x 10-4) and 1.0 M NaF

Acid Base Pt 2 So what is the point of a buffer?? It contains two species that are conjugates of one another. One is an acid, the other is a base. With both an acid AND a base, you have the ability to neutralize anything added and maintain a fairly constant pH.

Problems Calculate the pH of a solution by adding 0.30 mol of HC2H3O2 and 0.30 mol of NaC2H3O2 to enough water to make 1.0 L. (Ka = 1.8 x 10-5)

Problems 2) Calculate the pH of a solution containing 0.085 M of HNO2 and 0.10 M KNO2. (Ka = 4.5 x 10-4)

Another way to produce a buffer…. You could take one of the species and react it with a strong acid or strong base Stoichiometric reaction As long as you don’t completely consume the original substance, you still have some of that substance and you’ve stoichiometrically created the conjugate

Problems 3) Calculate [F-] and the pH of a soln that is 0.20 M HF and 0.10 M HCl.

A Better Way! (or at least a different way….) Can always use ICE or……… Henderson-Hasselbach Equation: shows relationship of variables in a buffer solution.

Henderson-Hasselbach pH = pKa + log([base]/[acid]) or pOH = pKb + log([acid]/[base])

Henderson-Hasselbach What is the pH of a buffered soln that is 0.12 M lactic acid(HC3H5O3) and 0.10 M sodium lactate? Ka = 1.4 x 10-4

Henderson-Hasselbach What is the pH of a buffered soln that is 0.12 M lactic acid(HC3H5O3) and 0.10 M sodium lactate? Ka = 1.4 x 10-4

Practice Calculate the pH of 0.500 L of a buffered soln of 0.50 M formic acid,HCO2H, and 0.70 M sodium formate before and after adding 10.0 mL of 1.00 M HCl. Ka = 1.8x10-4

Practice Using an acetic acid/sodium acetate buffer solution, what ratio of acid to CB would you need to maintain a pH of 5.00? Ka = 1.8x10-5

Buffer Capacity Amount of acid or base it can neutralize before pH begins to change… 1 M HC2H3O2 and 1M NaC2H3O2 versus 0.1 M will have the same pH but different capacity to be a buffer.

Buffers [H+] or pH is determined by 2 factors: Ka expression and value Ratio of [ ] CA or CB By adding OH- it will react and dec[acid] and inc [base]. By adding H+ it will react and dec [base] and inc [acid]

Optimal Buffering Problem: A chemist needs a solution buffered at pH 4.30 and can choose from the following acids(and their salts). Chloroacetic acid Ka 1.35 x 10-3 Proponoic acid Ka 1.3 x 10-5 Benzoic acid Ka 6.4 x 10-5 Hypochlorous acid Ka 3.5 x 10-8