Question

1). Two 20.0 mL samples, one 0.200 MKOH and the other 0.200 M CH3NH2, were titrated with 0.100 MHI. Answer each of the following questions regarding these two titrations.

a). What is the volume of added acid at the equivalence point for KOH?

b). What is the volume of added acid at the equivalence point for CH3NH2?

c). Predict whether the pH at the equivalence point for each titration will be acidic, basic, or neutral.

d). Predict which titration curve will have the lowest initial pH.

2). Potassium hydroxide is used to precipitate each of the cations from their respective solution. Determine the minimum concentration of KOH required for precipitation to begin in each case.

a). 1.2×10⁻² M CaCl2

3). Use the Henderson–Hasselbalch equation to calculate the pH of each solution:

a). a solution that contains 1.31% C2H5NH2 by mass and 1.20% C2H5NH3Br by mass

b). a solution that is 11.5 g of HC2H3O2 and 11.5 g of NaC2H3O2 in 150.0 mL of solution

Answer 1

1). Two 20.0 mL samples, one 0.200 MKOH and the other 0.200 M CH3NH2, were titrated with 0.100 MHI. Answer each of the following questions regarding these two titrations.

a). What is the volume of added acid at the equivalence point for KOH?

mmol of base= MV = 20*0.2 = 4 mmol

V = mmol/M = 4/0.1 = 40 mL of acid needed for each

b). What is the volume of added acid at the equivalence point for CH3NH2?

Same, since equivalence point is the same for KOH and CH3NH2. V= 40 ml

c). Predict whether the pH at the equivalence point for each titration will be acidic, basic, or neutral.

the pH when HI + KOH will be neutral, since strong acids/bases are used pH = 7

The pH will be slightly acidic for CH3NH2 since you have the conjugate

CH3NH3+ --< CH3NH2 + H3O+, which is acidic

d). Predict which titration curve will have the lowest initial pH.

the curve for weak base --> flat since there is buffer formation, the pH does not changes drastically with volume

the curve of strong base --> rapidly incresase in pH, when equivalence point is reached the pH changes very fast