Try this LC Chemistry Q1 Mock Question!

Hey! Please note this question is a medley of potential questions designed by augmenting previous iodine/thiosulfate q’s designed for you to self assess your knowledge! If you want to excel in LC chemistry, the code CHEM20 is valid for €20 off our LC Chemistry Revision Class on May 24th!

Q1: A solution of sodium thiosulfate (Na₂S₂O₃·5H₂O) was prepared by dissolving a known mass of

the solid in deionised water and making the solution up to 500 cm³ in a volumetric flask. This solution was titrated against 25.0 cm³ portions of a 0.050 M iodine solution.

The reaction is:

I₂ + 2S₂O₃²⁻→ 2I⁻ + S₄O₆²⁻

(a)

(i) Identify the reducing agent in this reaction.

(ii) Define the term reducing agent.

(6)

(b)

Sodium thiosulfate is not a primary standard.

Explain fully what is meant by a primary standard.

(6)

(c)

Pure iodine is almost insoluble in water.

Explain how iodine is brought into aqueous solution for this titration.

(3)

(d)

Describe how the sodium thiosulfate solution is prepared and made up to exactly 500 cm³ in a

volumetric flask.

(9)

(e)

A starch indicator is used in this titration.(i) At what stage is the starch added?

(ii) Describe the sequence of colour changes observed from the start of the titration to the end

point.

(9)

(f)

25.0 cm³ of 0.050 M iodine solution required 20.0 cm³ of sodium thiosulfate solution for

complete reaction.

Calculate:

(i) the number of moles of iodine in 25.0 cm³

(ii) the number of moles of sodium thiosulfate required

(iii) the concentration of the sodium thiosulfate solution in mol L⁻¹

(iv) the concentration of the solution in g L⁻¹ of Na₂S₂O₃·5H₂O (Mᵣ = 248)

(12)

(g)

A teacher notices that half of the class used deionised water while the other half used distilled

water in preparing and carrying out this experiment. The results obtained by the two groups

showed significant discrepancies.

(i) Which group of students would be expected to obtain more accurate results?

(ii) Explain your answer.

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QUESTION 1 – MARKING SCHEME BELOW (SCROLL)

QUESTION 1 – MARKING SCHEME

(a)

(i) sodium thiosulfate / S₂O₃²⁻ (3)

(ii) reducing agent:

substance that loses electrons / is oxidised / causes reduction (3)

(b)

PRIMARY STANDARD:

pure / stable / anhydrous (not hydrated) / no water loss (no efflorescence) /

not deliquescent / not hygroscopic / does not sublime / high molecular (molar) mass (Mᵣ) //

from which solutions of known concentration (molarity) can be made /

no need to standardise by titration / water soluble (3 + 3)

(c)

reaction with potassium iodide (KI) / iodide ions (I⁻) /

I₂ + I⁻

→ I₃⁻ / forms soluble triiodide (I₃⁻) / KI₃ (3)

(d)

DESCRIBE:

rinse from clock glass into beaker containing deionised water //

stir // dissolve //

pour (add) through funnel into volumetric flask //

add rinsings of beaker //

add deionised water until bottom of meniscus on (level with) mark //

read at eye level //

stopper and invert several times

ANY THREE: (3 × 3)

(e)(i)

when colour in conical flask is light (pale) yellow / straw coloured /

close to end point / when nearly all iodine used up (3)

(ii)

COLOURS:

red / brown / reddish brown / golden-brown / yellow-brown (3)

orange / yellow / light yellow / straw coloured (3)

blue-black / blue (3)

colourless [Do not accept ‘clear’] (3)

[Give marks if order correct even if one or more omitted.]

(f)

(i)

25 × 0.05 / 1000 = 0.00125 moles (3)

(ii)

0.00125 × 2 = 0.0025 moles (3)

(iii)

0.0025 / 20.0 = 0.000125 mol cm⁻³ (3)

0.000125 × 1000 = 0.125 M (3)

(iv)

0.125 × 248 = 31 g L⁻¹ (3)

[addition must be shown for error to be treated as slip]

(g)

(i)

distilled water (2)

(ii)

deionised water could contain non-ionic substances that could be oxidised or reduced /

could contain chlorine (oxidising agent) /

deionised water has had only ions removed /

distilled water is pure (purer, contains less impurities, contains no dissolved substances) (3)

[‘deionised water contains no ions’ is not acceptable]