5. Experiments 1,2,3,4 were performed with 25.0 mL of 4 monoprotic acids. The ac

Question

5. Experiments 1,2,3,4 were performed with 25.0 mL of 4 monoprotic acids. The acid molarity is unknown. They were each titrated with a monobasic base of 0.10 M. (alkali is another word for base) trong Acid-Strong Base Strong Acid-Weak Base PH 14 eak Acid -Strong Base Weak Acid- Weak Base a) Use the graphs to calculate the molarities of the four acids b) On each graph mark and write the value of the initial pH of the acid and the pli at equivalence. c) Why are the initial pH and equivalence points at different values even-though the concentration of each acid is the same? Explain your answer using the idea of strong and weak acids. 6. Look at the two graphs and explain why one of them has an S- shape and the other is an inverted S-shape. point Titration eurve of strong acid Titration ewve of strong bese CHCI) with a strong bave (NaOH) (NaOH] with strong acid (HE

Explanation / Answer

5. From the acid-base plots shown above

a) molarity of acid in 1, 2, 3 and 4

= 0.1 M x 25 ml/25 ml

= 0.1 M

b) initial pH in 1 and 2 = 0

initial pH in 3 = 2.6

initial pH in 4 = 2.3

Equivalence point pH for 1 = 7.0

Equivalence point pH for 2 = 4.8

Equivalence point pH for 3 = 9.2

Equivalence point pH for 4 = 7.0

c) the initial pH is of the strength of acid used. In case of strong acid we have complete dissociation of acid therefore pH is lower (high acidity).

Whereas, in case of weak acid we have partial dissociation of acid in solution, thereby the pH is higher (low acidity).

At equivalence point, strong acid-string base forms salt which is again completely dissociated in solution and has equal amounts of H+ and OH- in solution making the pH as 7.0.

In case of strong acid-weak base, the salt formed has a a weak conjugate acid of base which dissociates only to a small extent in solution, thereby the pH at equivalence point is lower (below 7.0)

In case of weak acid-strong base, the salt formed has a weak conjugate base of acid formed, which hydrolyzes to a small extent in solution giving free OH- in solution. That is solution is basic in nature at its equivalent point.

Finally in case of weak acid-weak base the salt formed has equal strength of conjugate forms of acid and base and thus gives same amount of solvation. pH thus seen is close to neutral 7.0.

6. In the two graphs, the first one is strong acid-strong base titration. Here, initial pH is lower (high H+) as all of acid is dissociated in solution. The pH increases (higher OH- added, lower H+ remained) as the titration proceeds. therefore the shape is S-type.

In the second case, titration of strong base (initial pH higher, high OH- concentration in solution) with strong acid is shown. Here the high OH- initially present is reduced over time by the addition of strong acid (H+) to solution. therefore, the pH decreases (low basicity, low OH-).

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