Question

(write clearly please.)

1. Determine the approximate time for the equivalence point; that is, for the biggest jump in PH in the steep vertical region of the curve (in other words the equivalence point is found in the middle of the vertical region of the titration curve. Use the experimental results to complete Table 1: Table 1 - Equivalence Point and Indicator Color Change Time of Indicator Color Time at Equivalence pH at the Change(sec) Point (sec) Equivalence Point 140 140 7.5 165 8.6 Initial Final PH pH 11.8 Acid-Base Combination HCI + NaOH HC HOA + NaOH HCI + NH,OH HC,H,O + NH,OH 1 165 3.9 12 140 140 5.0 0.9 8.8 155 155 7.5 4 10.5

the data for the table was found by looking at these 4 graphs:

use the data table and graphs to answer the following questions:

Answer 1

The major concept that is used in following answers is how the conjugate base/acid of an acid/ base behaves.

• Conjugate base/acid of strong acid/base are weak.
• Conjugate base/acid of weak acid/base are strong (hence conjugate ion can abstract proton or hydroxyl from water and shifts the pH towards acid or basic side.)

Q. 2a. Ans: Combination of strong acid with strong/weak base has initial value 1 or close to 1.

Q. 2b. Ans: Yeah, these are the expected results. we are measuring the pH  of solution in conical flask. So, when we havent added any base to it, the pH of acid is the initial pH.

Q. 3. Ans: pH at equivalence point is neutral for SA (strong acid) + SB (strong base) and WA (weak acid) + WB (weak base), acidic for SA+WB, basic for WA+SB. Yeah, these are the expected results. We can understand it by considering the product formed after reaction of acid and base.

• For strong acid and strong base HCl + NaOH = NaCl + H₂O, products are neutral pH (cation of strong base i.e. Na⁺ & anion from strong acid i.e. Cl^- are weak conjugate acid and base respectively and will not abstract proton or hydroxyl from water), hence pH @neutral.
• For SA and weak base HCl + NH₄OH = NH₄Cl + H₂O, Ammonium cation is conjugate cation of weak base i.e. strong acid ; hence it will abstract hydroxide from water (excess of proton) and make solution more acidic.
• For Weak acid and strong base CH₃COOH + NaOH = CH₃COONa + H₂O, Acetate anion is conjugate anion of weak acid i.e. strong base ; hence it will abstract proton from water (excess of hydroxyl ion) and make solution basic.
• For weak acid and weak base CH₃COOH + NH₄OH = CH₃COONH₄ + H₂O, Here both the conjugate anion and cation are coming from weak acid and base respectively. As both weak acid and base has almost similar k_a, the abstraction of proton and hydroxyl is almost similar and hence system remains neutral.

Q. 4. Ans. For SA + SB and WA + SB pH=9 on vertical region.

b) For alll the mentioned cases, time of the indicator color change and time at the equivalence point matches. Actually the color change of an indicator generally termed as end-point and it depends on how precisely the color change is observed. End point can vary depending upon various factor like concentration of indicator etc. Equivalence point can only be obtained when pHmetric machine is used and the above mentioned graphs are plotted.

Q. 7. Ans. For the case of strong acid and strong base indicator changing color at pH 5 or 9 works well. This can be understood by looking into the mentioned plot in the question. Both pH=5 and pH =9 falls into the vertical region of the curve, hence both type of indicator can change color during the sudden jump in pH.