Show all workout and answer all parts ( Im just reviewing for AP exams so its not graded, but I want it to help me.

Question

Show all workout and answer all parts ( Im just reviewing for AP exams so its not graded, but I want it to help me.

A new weak acid, HG, has been synthesized. The a titration of 0.115g of HG requires 16.00 mL of 0.10 M KOH solution to reach the equivalence point.

(a) Write the dissociation equation for HG in water and its equilibrium expression (K_a). 

(b) Write the net ionic equation for the titration reaction. 

(c) Calculate the molar mass of HG.

(d) After 8.00 mL of the base is added, the pH of the solution is 6.20. What is the value for the K_a of HG? 

(e) What is the value of K_b for the G^- ion? 

(f) What would be the effect on the pH if 500. mL of distilled water were added to the solution at the half-way point of the reaction? Justify. 

(g) Calculate the pH of the solution at the equivalence point (assume total volume is 16.00 mL). 

Answer 1

a. HG + H2O =====> H3O¹⁺ + G^1-

Ka = [H3O¹⁺] [G^1-] / [HG]

b. HG + OH^1- ====> G^1- + H2O

c. First calculate the # of moles of KOH to equivalence point.

0.1M/1000mL/L x 16 mL = 0.0016 moles which is equal to the # of moles of HG. 0.115 g HG / 0.0016 moles = 71.9 g/mole. Note: no volume for the acid was given.

d. Using the Henderson-Hasselbalch equation: pH = pKa + log [G^1-]/[HG].

Rearranging: pKa = pH - log[G^1-]/[HG]. At half the equivalence point,

[G^1-] = [HG], so pKa = pH = 6.2. Ka = antilog of -pKa = 6.31 x 10^-7

e. Kb = 10^-14 / 6.31 10^-7 = 1.58 x 10^-8

f. There would be no effect on the pH as the concentrations of the salt and acid would be diluted equally.

g. G^1- + H2O ⇔ HG + OH^1-. K_H = [OH^1-]² / [G^1-]

K_H = K_w/K_a = K_b = [OH^1-]² / [G^1-],

[G^1-] = 0.1M KOH / 1000mL x 16mL / 16 mL x 1000 mL = 0.1 M (the molarity of [G^1-] = the molarity of KOH added.

[OH^1-]² = K_H x [G^1-] = 1.58 x 10^-8 x 0.1 M, [OH^1-] = √1.58 x 10^-9 = 3.97 x 10^-5. [H¹⁺ ] = 10^-14 / 3.97 x 10^-5 = 2.52 x 10^-10 or pH = -logH¹⁺ ] = 9.60

There were a lot of calculations, and I tried to go step-by-step. Please check all calculations and let me know if any mistakes were made.