Question

PRE-LABORATORY QUESTIONS Fully answer these questions in your laboratory notebook before coming to lab. Show all work for numerical calculations. 1. Write out the chemical equation and equilibrium constant expression (Law of Mass Action) for the acetic acid / acetate ion buffer system. Derive the Henderson-Hasselbach equation for this system. 2. Calculate the pH of a buffer solution made by adding 1.3230 g of sodium acetate trihydrate (NaC,H,O, 3H,O) to 50.00 mL of 0.1652 M acetic acid. Assume there is no change in volume on adding the salt to the acid. PK for acetic acid is 4.74. 3. Buffer capacity refers to the amount of acid or base a buffer can "absorb" without a significant pH change. It is governed by the concentrations of the conjugate acid and base components of the buffer. A 0.5 M buffer can "absorb" five times as much acid or base as a 0.1 M buffer for a given pH change. In this problem, you begin with a buffer of known pH and concentration and calculate the new pH after a particular quantity of acid or base is added. You are given 75 mL of 0.70 M acetic acid/acetate buffer to test. The starting composition of the two major species are: Concentration of CH COOH: 0.250 M Concentration of CHCOO: 0.450 M a. Calculate the initial pH of the buffer. Clearly show all work required to arrive at your answer. b. You add 1.0 mL of 2.00 M HCl to the buffer. Calculate the molarity of 1,0' added as HCl, and the final molarities of acetic acid and acetate ion at equilibrium. What is the new value of pH? c. Now take a fresh 60 mL of the buffer and add 1.0 mL of 2.00 M NaOH. Using steps similar to those above, calculate the new pH of the solution

Answer 1

2 Chemical equation for the acetic - acid acetate ion buffer system can be written as .. Follows: CH3CO2H (aq) + troll - Hot (aa) + CH3CO2 (aa Acetic acid acetate .. Equilibrium constant expression for above reaction or equation can be given as Follows: Equilibrium constant (K) = [Hot] [CH3 Coz] ECH3CO2H] [H₂0] Since, concentration of solid or liquid is taken as unity inel [H20(4)] = 1 ; . So, Equilibrium constant (Ķ) = LH3ot) [CH3což J 7 [CH₃CO2H] can be we know that acid dissociation constant (ka) for above equation - given as follows: - Ka= [Hot] [CH₃což ] C [CH₃CO2H]

[Hot] = kax [CH3CO2H] [CH3což] Taking negative logarithm on both sides of above equation, we get - - - log[Hot] - - Log ( kax . ICH3CORH) [CH₂ což في حلم СъC CHзо - -CK Сс на он [CHg což j) sincet pH = -log([H3ot), pka = -log(ka) - pH = Pka - log ([CH3CO2H] [CH3cor] ! - pH = pka + Log (CChacož] [CH3CO2H] Hence, IPH = Pkat Log? CH 3 Coz] [CH3CORH Above equation is Henderson - Hassesbalah equation. Where. [CH3 Cor conjugate base LCHz Con. H acia