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6.4.3Quantitative Chemistry

Concentration of an Unknown Acid

5 min read 5 parts Lesson 10 of 12 1 quiz

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Section 1 of 5Apparatus and Setup

6.4.3 Concentration of an Unknown Acid

Accurate volumes and a known alkali concentration are the basis of this practical.

Pipette
  • A 25.0 cm³ volumetric pipette with a pipette filler measures the fixed volume of alkali exactly.
Burette
  • A 50 cm³ burette holds the unknown acid and delivers it in measured volumes.
Alkali
  • Sodium hydroxide solution of 0.100 mol/dm³ is the solution of known concentration in the conical flask.
Indicator
  • Methyl orange is added to the flask, and it changes from yellow to red or orange at the end point.

Section 2 of 5Method and Results

The method is repeated until two titres agree, then the mean of the concordant results is taken.

1
Pipette 25.0 cm³ of sodium hydroxide into a conical flask placed on a white tile, and add methyl orange.
2
Fill the burette with the unknown sulfuric acid and record the first reading to the nearest 0.05 cm³.
3
Run acid into the flask, swirling constantly, and stop at the first colour change to get a rough titre.
4
Repeat accurately, adding the acid drop by drop near the end point, until two concordant titres are obtained.

Section 3 of 5From Titre to Concentration

From Titre to Concentration
Equation
  • H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l), so one mole of acid reacts with two moles of alkali.
Mole ratio
  • The 1 : 2 ratio of acid to alkali must be used, because it changes the moles of acid obtained.
Conversion
  • Titre volumes must be converted from cm³ to dm³ by dividing by 1000 before calculating.
Conclusion
  • If less acid is needed than alkali for the same moles, the acid is more concentrated than the alkali.

Section 4 of 5Worked Example

Question

  • 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide neutralised a sulfuric acid of unknown concentration.
  • The mean titre of the acid was 15.00 cm³.
  • Calculate the concentration of the sulfuric acid.

  1. Moles of NaOH = 0.100 × 0.0250 = 2.50 × 10⁻³ mol.
  2. From H₂SO₄ + 2NaOH, the moles of acid = 2.50 × 10⁻³ ÷ 2 = 1.25 × 10⁻³ mol.
  3. Concentration of acid = 1.25 × 10⁻³ ÷ 0.01500 dm³ = 0.0833 mol/dm³.

Answer 0.0833 mol/dm³ (about 0.083 mol/dm³) sulfuric acid

Section 5 of 5Exam Checklist & Tips

6.4.3 – Key Points for the Exam

  • Standard solution A solution of accurately known concentration, such as the 0.100 mol/dm³ sodium hydroxide used here.
  • Volumetric pipette A pipette that delivers one fixed volume, such as 25.0 cm³, with high accuracy.
  • White tile A background placed under the flask so that the colour change at the end point is easier to see.
  • Mole ratio The ratio of moles of reactants in a balanced equation, used to convert between acid and alkali.

Common Mistakes

  • Using the 1 : 1 ratio for sulfuric acid, when the balanced equation shows a 1 : 2 ratio.
  • Forgetting to convert cm³ into dm³ before dividing moles by volume.
  • Including the rough titre when calculating the mean titre of concordant results.
  • Taking the concentration from the alkali volume instead of the acid titre volume.
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