Topic 8 · page 20 of 29

Core Practical 3: concentration of hydrochloric acid

Titrate a measured volume of hydrochloric acid with a standard solution of sodium hydroxide, using methyl orange, until you have concordant titres, and calculate the concentration of the acid.

13 min read8.21

Read first: Titration calculations

Aim and idea 8.21

To find the concentration of a solution of hydrochloric acid, by titrating it against a standard solution of sodium hydroxide of known concentration.

HCl(aq)+NaOH(aq)→NaCl(aq)+HX2O(l) \ce{HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)}

The ratio is 1 : 1. If you know the concentration and volume of the NaOH needed to neutralise a measured volume of the acid, you can find the concentration of the acid.

Method 8.21

PracticalTitrating hydrochloric acid with sodium hydroxide

  1. Rinse the burette with a little of the standard sodium hydroxide (0.100 mol dm⁻³) and fill it, using a funnel. Remove the funnel. Run solution through the tip to remove air bubbles. Record the initial reading to 0.05 cm³.
  2. Rinse the pipette with the hydrochloric acid. Use it to transfer 25.0 cm³ of the acid to a conical flask. Add 2–3 drops of methyl orange: the solution is red.
  3. Stand the flask on a white tile under the burette. Run in the alkali, swirling the flask, until the colour changes from red to orange. This is the rough titre.
  4. Repeat with fresh 25.0 cm³ portions of acid. Run in alkali to about 1 cm³ before the rough titre, then drop by drop, until the first permanent orange. Record the burette reading.
  5. Repeat until two or more titres agree within 0.10 cm³ (concordant).
Step Reason
Rinse the burette with NaOH, the pipette with HCl Any water left in them would dilute the solution and change the concentration
Rinse the conical flask with water only Water does not change the amount of acid in it, which is fixed by the pipette
Remove the funnel Drops from the funnel can add extra alkali to the burette reading
Remove air from the burette tip An air bubble in the tip leaves the burette, and the volume delivered is less than read
Swirl the flask So the reactants mix and the end point is not overshot
Add alkali drop by drop near the end point A single drop can pass the end point, so the titre is too high
White tile The colour change is easy to see
Read the bottom of the meniscus at eye level Avoids a reading error

Results and calculation 8.21

The results are illustrative (not measured data), for a typical good set.

Rough 1 2 3
Final burette reading / cm³ 25.20 25.05 24.50 24.80
Initial burette reading / cm³ 0.00 0.50 0.00 0.20
Titre / cm³ 25.20 24.55 24.50 24.60
  1. The rough titre is not used. Titres 1, 2 and 3 are concordant (24.50 to 24.60, a spread of 0.10 cm³).
  2. Mean titre = (24.55 + 24.50 + 24.60) ÷ 3 = 24.55 cm³
  3. n(NaOH) = 0.100 × 24.55 ÷ 1000 = 2.455 × 10⁻³ mol
  4. n(HCl) = 2.455 × 10⁻³ mol (1 : 1)
  5. c(HCl) = 2.455 × 10⁻³ ÷ 0.0250 = 0.0982 mol dm⁻³
  6. In g dm⁻³: 0.0982 × 36.5 = 3.58 g dm⁻³

Evaluating the practical 8.21

Source of error or uncertainty Effect Improvement
Overshooting the end point titre too high add drop by drop; use a half-drop at the end
Air bubble in the burette tip recorded volume too high check the tip before reading, and fill it first
Colour change not judged the same each time scatter between titres use a white tile; compare with a flask at the end point
Burette reading error (±0.05 cm³ per reading) random choose concentrations that give a large titre (about 25 cm³), so the reading error is a small fraction of it
Standard alkali absorbing CO₂ from the air concentration of NaOH slightly low keep the bottle stoppered; use a fresh solution

The uncertainty in this result is calculated on page 22.

Check yourself

  1. State why the burette is rinsed with the solution it is to contain, and the conical flask with water.
  2. Why is methyl orange used and what is the colour change at the end point?
  3. Titres of 25.30 (rough), 24.15, 24.20 and 24.10 cm³ are obtained for 25.0 cm³ of HCl against 0.0950 mol dm⁻³ NaOH. Calculate the mean titre and the concentration of the acid.
  4. State two ways to improve the accuracy of the end-point determination.
  5. A student finds that a titre was 0.30 cm³ too high in every run because of an air bubble in the burette tip. Is the error random or systematic? Explain.
  6. Calculate the concentration in g dm⁻³ of the HCl in question 3.

Answers cover these until you have tried the questions

  1. The burette is rinsed with the solution so that water left in it does not dilute the solution. The flask can be wet with water because the amount of acid in it is set by the pipette, not by the flask.
  2. A strong acid is titrated with a strong alkali, so methyl orange works. The colour changes from red to orange at the end point.
  3. Concordant titres: 24.15, 24.20, 24.10 (spread 0.10). Mean = 72.45 ÷ 3 = 24.15 cm³. n(NaOH) = 0.0950 × 0.02415 = 2.294 × 10⁻³ mol = n(HCl). c(HCl) = 2.294 × 10⁻³ ÷ 0.0250 = 0.0918 mol dm⁻³ (3 s.f.).
  4. Add alkali drop by drop near the end point; use a white tile; swirl; use a rough titre first; use a half-drop.
  5. Systematic: the same error in the same direction in every run, so the repeats agree with each other but all are wrong. Repeating the titration will not remove it.
  6. 0.09177 × 36.5 = 3.350 = 3.35 g dm⁻³. (Using the rounded 0.0918 gives the same, 3.35.)