Suggest an indicator which is suitable for determining the equivalence point of the titration reaction and state a reason on your choice.

Question

An unknown chemical (HB) was analysed. It is known that this chemical is a weak base. 1.94g of pure HB was completely dissolved in 32.00mL water. As it is titrated against 0.753M hydrobromic acid (HBr), 32.60mL was used to reach the equivalence point.

(a) Suggest an indicator which is suitable for determining the equivalence point of the titration reaction and state a reason on your choice.

(b) Please determine the molecular mass of the unknown chemical (HB).

Answer 1

The amount of titrant gives you the number of moles of HB present in the solution as HB + HBr ==> H2B¹⁺ + Br¹

0.753 moles / L or 0.753 moles / 1000 mL x 32.60 ml HBr = 0.0245 moles HBr = 0.0245 moles HB.

The molarity of the final titrated solution is 0.0245 / (32.0 + 32.6 mL)x 1000 = 0.379 M.

As this solution undergoes hydrolysis, we can calculate the pH of the final solution using the Kb.

HB + HOH =====> H2B¹⁺ OH^1-.

Kb = [OH^1-]² / [HB] = [OH^1-]² / 0.379 = 1.7 x 10^-9. Use the approximation that [HB] = 0.379 M.

[OH-] = √ (1.7 x 10^-9 x 0.379) = 2.54 x 10^-5 or pOH = 4.60, pH = 14 - 4.60 = 9.4

You would select an indicator that changes color around that pH. phenolphthalein will work.

As we determined that 0.0245 moles of HB are present and there is 1.94 g of HB, the molecular mass is

1.94 g / 0.0245 moles =79.2 g/mole