2.1

Question 1

2.1 Determination of the concentration of hydrochloric acid

Answer 1

For this Practical Activity, you will titrate a solution of sodium hydrogencarbonate against a solution of hydrochloric acid. You will use your results to work out the concentration of the hydrochloric acid.
When finding the exact concentration of an acid, you must use a standard solution of an alkali. This should be prepared by dissolving an accurate known mass of a base sodium hydrogencarbonate (NaHCO3) into distilled water and making this up to an accurately measured volume. A chemical that has a fixed chemical formula and does not absorb or lose moisture to the atmosphere must be used.
The indicator you will be using in this activity is methyl orange, which is red in acid and yellow in alkali. At the end point of the titration, the colour is orange.

Question 2

Aims

Answer 2

To determine the concentration of hydrochloric acid by titration with a standard solution of sodium hydrogencarbonate.

Question 3

chemicals needed

Answer 3

Solid sodium hydrogencarbonate, NaHCO3(s)
Dilute hydrochloric acid, HCl (aq)
(unknown concentration)
Distilled water
Methyl orange indicator

Question 4

Equipment

Answer 4

• Safety spectacles
• Burette
• White tile
• Pipette (25.0 cm3) and filler
• Clamp stand, with boss and clamp (for supporting the burette)
• Filter funnel
• Measuring cylinder (100 cm3)
• Glass rod
• Dropping pipette
• Volumetric flask and stopper (250 cm3)
• Wash bottle containing distilled (or deionised) water (about 300 cm3 will be required)
• Two conical flasks (250 cm3)
• Glass beaker (250 cm3 and 100 cm3) You will also need access to the following:
• Top-pan balance weighing to 0.01 g
• Dropping bottle containing methyl orange indicator

Question 5

Procedure part 1: Making up a standard 0.10 mol dm3 NaHCO3 solution

Answer 5

1. Weigh 2.10 g of solid sodium hydrogencarbonate. Transfer the solid to a clean, dry 100 cm3 beaker.
2. Add enough distilled water to dissolve the solid, stirring with a glass rod.
3. Transfer the solution to a clean 250 cm3 volumetric flask, using a funnel. Rinse the beaker and glass rod with distilled water and include the rinsings into the volumetric flask.
4. Make up to the mark with distilled water, insert the stopper, and invert several times to mix thoroughly.

Question 6

Procedure part 2: Finding the concentration of the HCl solution by titrating with your standard solution

Answer 6

1. Transfer your standard solution to a clean, dry beaker and label it.
2. Use a pipette and pipette filler to transfer 25.0 cm3 of this solution to a 250 cm3 conical flask.
3. Add a few drops of methyl orange to the flask.
4. Fill a burette with the hydrochloric acid provided.
5. Titrate the sodium hydrogencarbonate solution with the hydrochloric acid until the end point is reached. This is your trial titration.
6. Repeat your titration accurately until you have two consistent results. You may wish to keep the solution from your first accurate titration in order to achieve consistency by colour matching.
7. Ensure you have recorded all of your results.

Question 7

Analysis of results

Answer 7

1. Calculate your mean titre to two decimal places using your concordant results. You should indicate in your results which values you have used.
2. Calculate the amount in moles of NaHCO3 present in the 25 cm3 portion of your standard 0.10 mol dm3 solution.
3. The equation for the neutralisation reaction is shown below. Calculate the amount in moles of HCl present in the mean titre HCl (aq) + NaHCO3 (aq)  NaCl (aq) + H2O(l) + CO2 (g)
4. Calculate the concentration in mol dm3 of the hydrochloric acid. Give your answer to three significant figures.

Question 8

1. The balance has a maximum error of ±0.005 g in each reading. Calculate the percentage error in your mass of sodium hydrogencarbonate.

Answer 8

1. Calculation assuming the mass has been determined from two mass readings:
   % error = × 100 = 0.48%

Question 9

2. Explain why hydrated sodium carbonate and sodium hydroxide are not appropriate bases for making up a standard solution.

Answer 9

2. The substance used to make a standard solution must have an accurately known molar mass, so we know exactly how many moles we have dissolved in a given volume.Hydrated sodium carbonate loses water of crystallisation to the atmosphere (it is efflorescent), so its molar mass is variable.

Sodium hydroxide absorbs moisture from the air (it is deliquescent), so it also has a variable molar mass.

Question 10

3. The burette has a maximum uncertainty of ±0.05 cm3 in each reading. Calculate the percentage error in your first accurate titre.

Answer 10

3. This will depend on student results but using specimen results % error = × 100 = 0.44%

Question 11

4. The volumetric flask has a maximum error of ±0.3 cm3 and the pipette has a maximum uncertainty of ±0.04 cm3. Which of these gives the larger percentage error?

Answer 11

4. % error for volumetric flask = × 100 = 0.12%
   % error for pipette = × 100 = 0.16%
   Therefore, the pipette has the greater % uncertainty.