Precipitation Titration The table below contains conductometric titration data f

Question

Precipitation Titration The table below contains conductometric titration data for the following precipitation reaction. The titrant is the silver nitrate solution; the NaBr solution is the analyte. NaBr(aq) + AgNO_3(aq) rightarrow AgBr(s) + NsNO_3(aq) Would there be any visible signs that this reaction was occurring? If you answer yes, then describe the signs. Plot the data. Which ions are causing the conductivity at the following points: initially before the equivalence point at the equivalence point after the equivalence point Label the parts of the graph that correspond to the sections in part C, using the formulas for the ions. Clearly indicate the (vertical) boundaries for each section. Explain why we see a downward trend and then an upward trend in the conductivity as the titration progresses. Locate and label the equivalence point graphically (show this method on your graph) and with calculations (show/explain your work). More Acid-Base Titration Consider the pH curve for the titration of a weak base, B, as given below. A 0.562g sample of the weak base was dissolved in water to make a 50.00mL solution. The solution was titrated with 0.1994M HCl. B(aq) + HCl(aq) rightarrow HB^+ (aq) + Cl^-(aq) Determine the pH and volume of the equivalence point. Describe how you determined this. Calculate the molar mass of the base. Show your work. The correct molar mass of the base is 151.85g/mol; determine the %error in your molar mass. Determine the base dissociation constant, K_b, of the base. Show your work. What chemical indicator would work for this titration? Explain your choice. Site the reference used.

Explanation / Answer

#3 Precipitation titration

A. The formation of yellow precipitate in solution of AgBr clearly shows the progress of this reaction.

C. Ions responsible for conductivity at,

i. initially : Na+, Br-

ii. Before equivalence point : Na+, Br-

iii. at equivalence point : Ag+, Br-

iv. after equivalence point : Ag+, NO3-

E. We see a downward trend in conductivity plot initially due to precipitation of AgBr, which reduces the number of ions in solution. Once the solution becomes saturated we have excess of AgNO3 in the solution, which dissociate into ions and thus the conductivity rises after this point.

#4 Acid-base titration

A. pH at equivalence point = 6

Volume of HCl added at equivalence point = 18.5 ml

B. molar mass of base = 0.562 g/0.1994 M x 18.5 ml

                                     = 152.35 g/mol

C. % error in molar mas = (152.35 - 151.85) x 100/151.85 = 0.33%

D. from the graph,

pKb = pH at half equivalence point = 10.2

pKb = -log[Kb] = 10.2

base dissociation constant Kb = 6.31 x 10^-11

E. Methyl orange which shows color change around same pH as the equivalence point would be the best indicator for the given system.

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