Module 6.3 Quantitative Analysis

The diagram represents the titration curve for a reaction between a particular acid an particular base.

Which indicator would be best for this titration?

• Martius yellow (pH 2.0 - 3.2)

• Magdala red (pH 3.0 - 4.0)

• Isopicramic acid (pH 4.0 - 5.6)

• Cresol red (pH 7.2 - 8.8)

Which of the following equations best represents the reaction described by the titration curve?

• NH₃(aq) + HCl(aq) → NH₄Cl(aq)

• NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

• NH₃(aq) + CH₃COOH(aq) → CH₃COONH₄(aq)

• NaOH(aq) + CH₃COOH(aq) → CH₃COONa(aq) + H₂O(l)

A weak base is titrated with 1.0 mol L⁻¹ aqueous HCl. The pH curve is shown.

At which pH value would the solution be most effective as a buffer?

• 5

• 7

• 8

• 9

A student used the following method to titrate an acetic acid solution of unknown concentration with a standardised solution of dilute sodium hydroxide.

• Rinse burette with deionised water.

• Fill burette with sodium hydroxide solution.

• Rinse pipette and conical flask with acetic acid solution.

• Pipette 25.00 mL of acetic acid solution into conical flask.

• Add appropriate indicator to the conical flask.

• Titrate to endpoint and record volume of sodium hydroxide solution used.

Compared to the actual concentration of the acetic acid, the calculated concentration will be

• lower.

• higher.

• the same.

• different, but higher or lower cannot be predicted.

Which of the following describes what happens when a solution of a weak acid is diluted? (Assume constant temperature.)

• Ka decreases and the extent of acid ionisation increases.

• Ka decreases and the extent of acid ionisation decreases.

• Ka remains the same and the extent of acid ionisation increases.

• Ka remains the same and the extent of acid ionisation decreases.

This titration curve is produced when an acid is titrated with a sodium hydroxide solution of the same concentration.

How many acidic protons does this acid possess?

• 1

• 2

• 3

• 4

The trimethylammonium ion, [(CH₃)₃NH]⁺, is a weak acid. The acid dissociation equation is shown.

[(CH₃)₃NH]⁺(aq) + H₂O(l) ↔ H₃O⁺(aq) + (CH₃)₃N (aq) Ka = 1.55 x 10^-10

At 20°C, a saturated solution of trimethylammonium chloride, [(CH₃)₃NH]Cl, has a pH of 4.46.

What is the Ksp of trimethylammonium chloride?

• 1.26 × 10^–9

• 7.76

• 60.2

• 5.01 × 10¹⁰

When a solution of a primary standard is prepared for titration, which of the following is required?

• A burette

• A balance

• An indicator

• A condenser

A 25.00 mL sample of 0.1131 mol L⁻¹ HCl(aq) was titrated with an aqueous ammonia solution. The conductivity of the solution was measured throughout the titration and the results graphed.

What was the concentration of the ammonia solution?

• 0.0452 mol L⁻¹

• 0.189 mol L⁻¹

• 0.283 mol L⁻¹

• 0.690 mol L⁻¹

Cyanidin is a plant pigment that may be used as a pH indicator. It has four levels of protonation, each with a different colour, represented by these equilibria shown in the image.

The graph shows the relative amount of each species present at different pH values.

What colour would the indicator be if added to a 0.75 mol L⁻¹ solution of hypoiodous acid, HIO (pKa = 10.64)?

• Red

• Colourless

• Purple

• Blue