Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

Solid W is an impure organic acid, $\text{H}_3\text{A}$. To find the mass of $\text{H}_3\text{A}$ present in a sample of W, a student carries out a titration using $0.100\ \text{mol dm}^{-3}$ sodium hydroxide, NaOH. The student: step 1 puts the sample of W into a beaker step 2 adds 100 cm$^3$ of distilled water to the beaker with a measuring cylinder step 3 mixes until all of the solid has dissolved completely step 4 gives this mixture the label X step 5 uses a pipette to transfer 25.0 cm$^3$ of NaOH into a conical flask and adds five drops of phenolphthalein indicator step 6 pours X into the burette step 7 runs X into the conical flask until the solution just alters colour step 8 carries out the titration two further times.
(a)[1]

Explain why distilled water, rather than tap water, should be used in this experiment.

(b(ii))[1]

Use the best titration readings (✓) to work out the average volume of X used.

(c)[1]

Calculate the number of moles of NaOH in 25 cm$^3$ of $0.100\ \text{mol dm}^{-3}$ NaOH.

(d)[1]

The reaction equation for $\text{H}_3\text{A}$ with NaOH is shown. $\text{H}_3\text{A} + 3\text{NaOH} \rightarrow \text{Na}_3\text{A} + 3\text{H}_2\text{O}$ Calculate the number of moles of $\text{H}_3\text{A}$ that react with 25.0 cm$^3$ of $0.100\ \text{mol dm}^{-3}$ NaOH.

(e)[1]

Calculate the number of moles of $\text{H}_3\text{A}$ in the sample of W.

(f)[1]

The relative molecular mass of $\text{H}_3\text{A}$ is 210. Calculate the mass of $\text{H}_3\text{A}$ in the sample of W.

(g)[2]

In step 2 a measuring cylinder is used to add 100 cm$^3$ of distilled water to the beaker. Explain why using the measuring cylinder makes the volume of distilled water used inaccurate. Suggest an improvement.

(h(i))[1]

Before the student carries out step 6 the burette is rinsed with distilled water and then with X. Explain why the burette is rinsed with distilled water.

(h(ii))[2]

Suggest and explain the effect on the titration results if the burette is not rinsed with X after rinsing with distilled water.

Worked solution & mark scheme

This 14-mark question has a full step-by-step worked solution and mark scheme. One marking point: “tap water contains dissolved substances / minerals / impurities” …

  • Full mark scheme, point by point
  • Step-by-step worked solution
  • Write your answer & get it marked instantly by AI