STUDENT SPACE · TOPIC 5

Titration and concentration

Use a burette and pipette, then calculate concentration from a titre.

The essentials

Use a volumetric pipette to put a known alkali volume in a flask. Add a few drops of a suitable indicator.
For HCl added to NaOH with thymolphthalein, the colour changes from blue to colourless.
Titre = final burette reading − initial reading. Repeat to obtain closely agreeing titres.

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

Why repeat a salt-preparation titration without indicator?

Need a hint for Question 1?

Convert cm³ to dm³ and write the mole ratio before calculating the unknown.

Check Question 1

The indicator would contaminate the salt; use the measured reacting volumes without it.

A burette starts at 1.20 cm³ and ends at 24.80 cm³. Calculate the titre.

Need a hint for Question 2?

Delivered volume = final reading − initial reading.

Check Question 2

24.80 − 1.20 = 23.60 cm³.

25.0 cm³ of 0.200 mol dm⁻³ NaOH reacts with 20.0 cm³ H₂SO₄. Use H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O to find c(H₂SO₄).

Need a hint for Question 3?

Convert volumes to dm³ and apply the 1:2 acid-to-alkali ratio.

Check Question 3

n(NaOH) = 0.00500 mol; n(H₂SO₄) = 0.00250 mol. c = 0.125 mol dm⁻³.

Use Lesson 5 in your worksheet for written practice. Your teacher will discuss those answers in class.