When a strong base is gradually added dropwise to a weak acid, the pH changes at

Question

When a strong base is gradually added dropwise to a weak acid, the pH changes at each addition. When the appropriate quantity of base has been added to react with all of the pH changes sharply indicating the endpoint the titration. A plot of pH versus volume of base added gives what is known as a titration curve. Consider the titration of 25.00 mL of o 1000 M acetic acid with 0.1000 M NaOH. a. Write the equation for the titration reaction. b. Determine the volume of NaOH solution required to reach the endpoint. 25 00ml c. The chart below has entries for several steps along the titration curve. To calculate the pH at each step, you first must understand the chemistry at that step, and then you can decide the appropriate method to calculate the pH. For each volume listed use the appropriate equation to calculate the solution [H_3O^+] and pH. Show all your work.

Explanation / Answer

a) equation for the titration is

CH3COOH + NaOH --------------> CH3COONa + H2O

b)At the end point

mmoles of acid = mmoles of base

25 x0.100 = V x 0.100

Thus volume of NaOH required to get the end point = 25 mL

3) When no NaOH is added the solution has only weak acid , thus

HA +H2O -------------> H3O+ + A-

C 0 0 initial

C(1-x) Cx Cx after ionizaiotn

thus [H3O+] =Cx

and Ka = Cx2 after approximating 1-x = 1

Thus [H3O+] = (KaC)1/2

and pH = 1/2 [pka - logC]

Before reaching the end point , at all volumes of NaOH added the solution behaves as a buffer constituting the weak acdi ,acetic acid and its conjugate base sodium acetate .

Thus the pH of the buffer is given by

pH = pKa + log [Conjugate base]/[acid]

and [H3O+] = ka.[conjugate base] /[acid]

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