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How much acid is there in a solution

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How much acid is there in a solution?

When a metal is extracted from its ore, one by-product is sulphur dioxide, which can be used to make more useful by-products such as sulphuric acid. Sulphur dioxide dissolves into water vapour in the atmosphere to make “acid rain”, which contains sulphuric acid and can cause environmental damage.

The sulphuric acid in the solution of “acid-rain” has a concentration of about 0.05 mol-1 and 0.15 mol-1. The task is to find out the exact concentration by using a titration to then find out how much acid is in the solution. This is can only be done by using a solution with a known concentration

The Brønsted-Lowry theory characterises an acid as a proton donor or has the ability to donate H+ ions, whereas a base is a substance that accepts H+ ions. Strong acids and bases are substances that for ions easily in water and in dilute solutions they are made entirely of ions. Weak acids and bases are substances which form solutions which are partially in form of ions.

These curves show the pH change when different concentrations of bases are added to different concentrations of acids and vice versa. The curve that applies to this investigation is the “Strong acid to weak base” curve.

Indicators

Indicator

pH range

Litmus

5 - 8

Methyl Orange

3.1 – 4.4

Phenolphthalein

8.3 – 10.0

Indicators are weak acids which change colours within a certain pH range, for example:

From the titration curves the best indicator to use would be Methyl orange, as the pH range lies on the line, which has the same volume, it should have a distinct colour change from yellow to red. If the other indicators, litmus or phenolphthalein were to be used, the results are likely to be inaccurate as there are a wider range of volumes at which the pH changes at, in relation to the titration curve.

As indicators are weak acids, only a few drops are needed, if 5 or 10ml were added, it would cause the solution being titrated to be slightly more acidic and may effect results.

Group I carbonates are highly soluble and are also thermally stable, so they won’t decompose when heated.

The reaction between sodium carbonate and sulphuric acid will be a neutralisation reaction, forming a soluble salt, water and carbon dioxide:

Na2CO3(aq) + H2SO4(aq)→ Na2SO4 + H2O + CO2

The standard solution will be 250ml of 0.1mol-1 of Na2CO3. This will give a known solution, which can then be used to calculate the concentration of the acid rain. The standard solution will allow a number of titrations to be done; the concentration will stay constant in all samples of the solution and therefore will make the calculations more accurate.

Once this standard solution has been made it will be used in a titration to work out the concentration of the sulphuric acid.

Calculations:

RMM of Na2CO3 = 2(23) + 12 + 3(16)
                = 46 + 12 + 48

                = 106   106g = 1 mole
                            10.6g = 0.1 moles

If 10.6g of sodium carbonate is dissolved in 1dm-3 of water, it would produce a solution of 0.1 moles, as we only need 0.25 dm-3 of solution, we only need a quarter of this mass.

10.6 x 0.25 = 2.65g

To measure the sodium carbonate, place a volumetric flask onto mass balance, which measures to 2 decimal places; then using a spatula add the sodium carbonate. Stop adding sodium carbonate when it weighs around 2.65g. Using distilled water wash down the neck of the bottle and the top, so that all the sodium carbonate will dissolve.

The actual measurement of sodium carbonate was 2.64g

To ensure that all equipment is clean, rinse it out with distilled water, as it won’t affect the solution.

To ensure that all the sodium carbonate has all dissolved, shake the volumetric flask with a lid vigorously. If there is no more solid at the bottom of the flask, then all the sodium carbonate has dissolved, the solution can then be made up to 250ml so that the meniscus sits on the line.

The meniscus is caused by water tension, as there is an imbalance in forces along the surface of the water. Molecules below the surface pull the surface downwards.

To make sure that all the equipment is clean, it should be rinsed with the solution that is going to be put in it, so that any if any other substance that was left in it won’t affect the results or dilute any of the solutions. The burette should be rinsed with the solution of sulphuric acid, as the acid will be titrated against the base. The bulb pipette should be rinsed with the base and the conical flask can be rinsed with distilled water, as it won’t affect the base that is going to be put in it.

To fill the burette, it is easier and safer to place the burette in a clamp stand and place it on the ground. By making sure that the taps is closed and using a funnel, pour the sulphuric acid into the burette, after it is full take the funnel out, so that it won’t drop any last bits of sulphuric acid and cause the results to be inaccurate. To get rid of the air lock at the bottom of the burette, place a beaker under the bottom of the burette and turn the tap on so that sulphuric runs through and fills the air lock.

Using a 25cm3 bulb pipette, fill it up past the line with the standard solution. By taking the line on the pipette to eye level, gently let some of the standard solution out, so that the meniscus sits on the line to make the measurement accurate. This can then be placed in the conical flask, ready to be titrated.  

Titration

  • Note down the initial reading of the burette
  • Turn the tap gently so that some of the sulphuric acid goes into the conical flask, then close it
  • At the same time, swirl the conical flask
  • Keep adding sulphuric acid slowly until there is a slight colour change
  • Once there has been a colour change, add the sulphuric acid drop by drop and continue to swirl the conical flask
  • If the colour changes with a single drop, stop titrating and close the tap
  • Using a wash bottle wash down the sides of the conical flask to make sure that all of sulphuric acid has been added.
  • Note down the final reading of the burette and work out the titre added.

By doing a “dry run” you can find out approximately how much acid to add, so that when the real attempts are done they can be done quicker as you can add a certain amount of acid with no colour change.

There will be enough standard solution to do about nine titrations. Once there are concordant titres, which are the same or are within 0.1cm3 of each other, no more repeats will need to be done.

Results will be shown on a table, with the final reading and initial reading noted and the titre added, which is the difference between the final and initial readings.

Preliminary

1

2

3

4

Final Reading

Initial Reading

Titre Added

Risk Assessment

  • Sulphuric is a strong toxic and corrosive acid. Protective eyewear must be worn and if it comes in contact with skin it should be washed off with plenty of water.
  • Sodium Carbonate is a weak base, which is slightly corrosive. Care should be taken if it comes in contact with skin and should be washed off with water.
  • Care should be taken when using the glassware and make sure that any broken glass is swept away as to not cause injury
  • Long hair should be tied up so it does not restrict vision or come in contact with harmful substances.

Analysis

Preliminary

1

2

3

4

Final Reading

39.40

39.80

39.50

Initial Reading

0.00

0.30

0.00

Titre Added

39.40

39.50

39.50

As the highlighted titres are concordant titres, there was no need to do any further titrations. They were also both within 0.1 of the preliminary titration. From the results, the concentration of the sulphuric acid can be calculated.

Na2CO3(aq) + H2SO4(aq)→ Na2SO4 + H2O + CO2

       1         :       1

As the stoichiometry of the reaction is one to one, therefore the number of moles of sodium carbonate is equal to the number of moles of sulphuric acid.

No. of moles of sodium carbonate (mol) = Volume (dm3)  x   Concentration (mol/dm3)

As 2.64g of sodium carbonate were used, the concentration of the solution is:

2.64 ÷ 0.25 = 10.56 g

10.56 ÷ 106 = 0.099 moles

The sample of sodium carbonate used was 25 cm

Therefore:

Volume (dm3) = 25 ÷ 1000 = 0.025 dm3

No. of moles of sodium carbonate = 0.025 x 0.099

                                   = 0.00248

The number of moles of sulphuric acid is also equal to 0.00248. By having the number of moles and the volume added, the concentration can be worked out.

Volume of sulphuric acid (dm3) = 39.50 ÷ 1000 = 0.0395

Concentration = No. of moles ÷ Volume

Concentration (mol) = 0.00248 ÷ 0.0395
                      = 0.062 moles

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