CHE 202-113 Exam 4 Mar 28 Spring 2018 Graph for Questions 17,18 The following pl
ID: 1034326 • Letter: C
Question
CHE 202-113 Exam 4 Mar 28 Spring 2018 Graph for Questions 17,18 The following plot shows two titration curves, each representing the titration of 50.00 mL of 0.100 M acid with 0.100 M NaOH. 14 12 10 0 200 460.0 80100.0 mL of 0.100 M NOH added 17) Which point a-d represents the equivalence point for the titration of a B) point b strong acid? 17) A) point a C) point c D) point d a-d represents the equivalence point for the titration of a weak acid? 18) Which point A) point a B) point b C) point 18) D) point dExplanation / Answer
17) The equivalent point for the titration of a strong acid with?
Ans: D) point d
When the analyte is a strong acid such as HCl and the titrant is NaOH which is a strong base. When we start plotting the pH of the analyte against the volume of the NaOH that we are adding from the burette we get a curve with the equivalent point at d.
Before the addition of NaOH, the pH of the analyte is low as it contains H3O+ due to the dissociation of HCl. With the dropwise addition of NaOH the analyte is still due to the predominance of H3O+ions. After the point b, complete neutralization takes place by the addition of OH- ions. At the equivalence point, no of moles of NaOH equals moles of HCl analyte and thus the pH=7. With further addition of NaOH pH starts becoming basic.
18) The equivalence point for the titration of a weak acid?
Ans: C) point c
Let's assume the analyte to be a weak acid such as acetic acid and the titrant to be a strong base such as NaOH. As we start plotting the pH of the analyte against the volume of NaOH we get titration curve ac.
With no NaOH added the pH of the analyte is low due to the presence of H3O+ present due to the dissociation of the acetic acid. But acetic acid is a weak acid so the starting pH is lesser than the strong acid like HCl. Due to the addition of NaOH the H3O+ consumed by the OH- ions produced due to the dissociation of NaOH. After the point 'a' the H3O+ ions are neutralized by OH- ions and we get a steep curve going up. At the halfway up curve which is called the equivalence point the H3)+ ions are completely neutralized by OH- ions. beyond the equivalence point, the pH excess is due to the presence of excess OH- ions due to the addition of NaOH.