Question

The graph above shows the titration curves of four monoprotic acids of varying strengths. All of the acids start out at 0.1 M concentration and 25.0 mL volume. Choose all of the correct statements about these titration curves from the choices below. 

Answer 1

Titration of strong acid vs strong base (NaOH)

HA. +. NaOH.   . NaA. + H_{​​​​​​2}​​​​​O

1) pH increases slowly
2) Steep increase in pH due to unreacted of NaOH as all the HCl present is neutralized. Solution become neutral or become basic.
3) Slow increase in pH

Titration of weak acid vs strong base ( NaOH)

HA. +. NaOH. . NaA. +. H_{​​​​​​2}​​​​​O

1) Firstly, a sharp increase in pH is observed at the beginning which is due to formation of anion i.e a conjugate base of the weak acid (common ion) that reduces dissociation of the weak acid.
2) Now solution contains a weak acid with it's strong conjugate base which will act as a buffer. This solution changes the curve. This kind of flat curve is observed until the base overcomes the buffers capacity.

3) This curve again changes steeply prior to equivalence point.
At the equivalence point the pH is greater then 7 because all of the acid (HA) has been converted to its conjugate base (A-) by the addition of NaOH and now the equilibrium moves backwards towards HA and produces hydroxide, that is:
A^{​​​​​​-.} + H₂O ⇌HA. +. OH^-

^{​​​​​}Note:

a) The steep increase in case of strong acid is over a long range of pH as compared to weak acid.

b) When a strong acid vs strong base titration is performed the end-point occurs at 7.

c) When weak acid vs strong base titration is performed the end-point occurs at pH greater than 7.

c) Weak acid vs strong base : The indicator phenolphthalein is used that changes color at high pH.

Statement I) As we can see the steep nature of the graph is more in acid D than C, B or A showing acid D is strongest of all. Statement I is true.

Statement II)

In the middle of curve B, the gradual flat curve shows half-neutralization means at this point, [HA] =. [A] where pH=pKa. As you can see this is occuring at around 6. So statement II is true.

Statement III) It's true and can be seen by the curve also. Equivalence point is the mid point in the steep nature of the curve. pH of strong acid vs strong base is around 7 whereas for weak acid it is more than 7 and depends upon strength of the acid, pKa and concentration of the acid, HA.

Statement III is true

statement IV) acid A is the weakest of all. Weak acid require indicator which change color on higher pH side. So indicator should be acidic in nature as phenolphthalein. So statement IV is true

statement V) it is also true. As seen by steep nature of the curve. Lessor the steepness, weaker the acid.

Statement VI) it is false.

As concentration of conjugate base will be high when 25 ml of NaOH is added.

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

pH can't be equal to pKa. It will be higher.