4.10 Required Practicals: Chemical Calculations

A free GCSE Chemistry lesson for AQA (8462) from OBA School: aQA required practical: titration – finding the volumes of acid and alkali that react (Higher tier: calculating a concentration). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Plan and carry out an accurate titration of a strong acid against a strong alkali
• Identify the dependent and control variables and assess the risks
• Process titration results: titres, concordant results, mean titre and uncertainty
• (Higher tier) use the mean titre to calculate an unknown concentration
• Explain how errors affect the titre and suggest improvements

Key words

• Accurate: Close to the true value.
• Precise / concordant: Repeat measurements that are close together. In titrations, titres within 0.10 cm³.
• Uncertainty: The range within which the true value is likely to lie. Estimate it as half the range of the repeat results.
• Systematic error: An error that shifts every result in the same direction, e.g. always reading the top of the meniscus.

Aim and equipment

Aim: to find the volume of hydrochloric acid that exactly neutralises 25.0 cm³ of sodium hydroxide solution, and (Higher tier) to use this volume to calculate the concentration of the alkali.

• 25.0 cm³ pipette + pipette filler; Use measure the alkali accurately and safely
• 50 cm³ burette, clamp stand, small funnel; Use hold, add and measure the acid (remove the funnel before titrating)
• Conical flask, white tile; Use mix the solutions; see the colour change
• Indicator: phenolphthalein or methyl orange; Use show the end point with a sharp colour change
• Eye protection; Use protect eyes from splashes of acid and alkali

Method

1. Rinse the pipette with a little sodium hydroxide and the burette with a little hydrochloric acid.

2. Use the pipette and filler to transfer 25.0 cm³ of sodium hydroxide to a conical flask (bottom of the meniscus on the line; let it drain and touch the tip on the flask – do not blow out the last drop). Add 3 drops of indicator.

3. Fill the burette with acid (below eye level, using a funnel). Run a little out to fill the tip and remove air bubbles. Remove the funnel and record the initial reading.

4. Place the flask on a white tile. Add acid while swirling. Near the end point add it drop by drop.

5. Stop when the indicator changes colour permanently. Record the final reading. Titre = final − initial.

6. Do a rough titration, then repeat accurately until you have at least two concordant titres.

Variables and risk assessment

• Dependent; In this practical volume of acid needed to neutralise the alkali (the titre)
• Control; In this practical volume of alkali (25.0 cm³); concentration of acid and alkali; type and number of drops of indicator; temperature

• Sodium hydroxide (irritant/corrosive) and hydrochloric acid (irritant); Risk damage to eyes or skin; Control measure wear eye protection; wash splashes off with plenty of water
• Glass burette and pipette; Risk breakage and cuts; Control measure fill the burette below eye level; clamp it securely; never pipette by mouth

Processing the results

Worked example: mean titre and uncertainty

mean = (22.45 + 22.55) ÷ 2 = 22.50 cm³
uncertainty = half the range of concordant titres = (22.55 − 22.45) ÷ 2 = ±0.05 cm³

Answer: 22.50 ± 0.05 cm³

Worked example: (Higher tier) concentration of the alkali

NaOH + HCl → NaCl + H₂O. The acid is 0.100 mol/dm³.
moles HCl = 0.100 × 0.02250 = 0.002250 mol = moles NaOH
concentration NaOH = 0.002250 ÷ 0.0250 = 0.0900 mol/dm³
in g/dm³: 0.0900 × 40 = 3.60 g/dm³

Answer: 0.0900 mol/dm³ (3.60 g/dm³)

Sources of error and improvements

• Overshooting the end point; Effect on titre too large; Improvement add acid drop by drop near the end point; use the rough titre as a guide
• Air bubble in the burette tip that comes out during the titration; Effect on titre too large; Improvement run some acid through the tip before taking the initial reading
• Burette rinsed with water, not acid; Effect on titre too large (acid diluted); Improvement rinse with the acid first
• Conical flask wet with distilled water; Effect on titre no effect; Improvement none needed – the moles of alkali do not change
• Reading the meniscus from above or below; Effect on titre random or systematic error; Improvement read the bottom of the meniscus at eye level

Exam tip: If an error makes the titre too large, the calculated concentration of the alkali will also be too large.

Common mistakes

• Calling the volume of alkali the independent variable. It is kept constant – it is a control variable.
• Using all the titres (including the rough one) to calculate the mean.
• Writing burette readings as 25.3 or 25.355. Record to 2 decimal places, ending in 0 or 5.
• Saying a wet conical flask gives a wrong answer. Water does not change the number of moles of alkali.

Revision questions

Try these, then check your answers below each one.

Question 1. In the titration required practical, what is the dependent variable? (Choose from: The volume of acid needed to neutralise the alkali, The volume of alkali in the flask, The type of indicator, The concentration of the acid.)

Answer: The volume of acid needed to neutralise the alkali

Working: The titre (volume of acid added from the burette) is measured. The volume of alkali, acid concentration and indicator are kept the same (control variables).

Question 2. True or false? Rinsing the conical flask with distilled water before adding the alkali changes the titre.

Answer: False

Working: False. The pipette still delivers the same moles of alkali, so the same volume of acid is needed.

Question 3. An air bubble in the tip of the burette comes out during the titration. What is the effect on the recorded titre? (Choose from: It is too large, It is too small, It is unchanged, It cannot be measured.)

Answer: It is too large

Working: The burette reading changes by the volume of the bubble as well as the acid, so the recorded titre is too large.

Related lessons

• Previous lesson: 4.9 Volumes of Gases
• Next lesson: 5.1 The Reactivity Series
• In this chapter: 4.1 Relative Masses and Moles
• In this chapter: 4.2–4.3 Equations and Calculations
• In this chapter: 4.4 The Yield of a Chemical Reaction

Continue in the full course

This lesson is section 4.10 (ARPCH0410) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.