Finding out how much acid there is in a solution

To make a concentration of 1 mol dmˉ³ you would need 106g of Na2CO3

To make the desired concentration of 0.1 mol dmˉ³ you would need 10.6g of Na2CO3 (106/10 = 10.6)

Therefore to make the concentration of our 250cm³ 0.1 mol dmˉ³ you would need 2.65g of Na2CO3 (10.6/4 = 2.65)

Therefore I will need 2.65g of Na2CO3 to be dissolved to make the solution.

Procedure

1. Using a spatula transfer exactly 2.65g of the solid sodium carbonate to 250cm³ of distilled water in the beaker.
2. Stir to dissolve the solid using the stirring rod.
3. Transfer the solution into the volumetric flask
4. Insert a stopper to prevent leakages
5. Set aside the solution ready for titration

Using this standard solution of sodium carbonate we may now plan for the actual titration.

Titration

Apparatus

• Safety goggles
• Clamp and stand
• Beaker (100cm³)
• Pipette (25cm³)
• Conical flask (250cm³)
• Methyl Orange Indicator
• Distilled water
• Burette
• White Tile
• Funnel

Diagram

Procedure

1. Using the funnel I will rinse out the burette with some sulphuric acid and then fill it with the unknown concentration sulphuric acid.
2. Using the pipette I will rinse and then transfer some of the solution of sodium carbonate into a clean conical flask.
3. I will add 2-3 drops of methyl orange indicator solution.
4. I will then run the sulphuric acid solution from the burette into the flask, swirling until the solution just turns pink. At that point I will stop.
5. This reading will be used a rough trial to help me get used to the equipment.
6. I will then continue to do this carefully until I have 2 readings within 0.1cm³ of each other.

For safety reasons, before I start the procedure I will first look at the risks involve with carrying out this experiment. I have obtained this information from textbooks and hazard cards.

The chemicals I am using have several risks associated with them:

Sodium carbonate

Although the solution I am using is very dilute I should still be aware that contact with eyes, skin and clothing must be avoided. As such I am to be wearing goggles and a lab coat. Also any slippages would cause the area to become slippery and dangerous, if they are not cleaned up quickly.

Methyl orange

The same precautions apply for this as did with the sodium carbonate.

Sulphuric acid

Highly concentrated acids react vigorously with water. The concentration of my acid is no more than 0.15 mol dmˉ³, which is therefore not very harmful. However, I must still avoid contact with eyes, skin and clothing and I must clean up any spillages quickly.

My plan for this acid-alkali titration should prove to be both precise and give reliable results, provided all chemicals are measured accurately and correctly.

The percentage errors associated with some of the equipment will be taken into account during the evaluation.

Results

Calculations

To calculate the concentration of the sodium carbonate solution used:

2.65g solid anhydrous sodium carbonate

Molar mass of Na2CO3 = 2 x 23 + 12 + 3 x 16 = 106g

Moles Na2CO3 = 2.65/106 = 0.0250 moles (3sf)

Combined with 250cm³ water = 0.25 dm³

Therefore, concentration in mol/dm³ = 0.025/0.25 = 0.100moldm³ (3sf)

Concentration of acid solution:

Equation of neutralisation of sulphuric acid with sodium carbonate:

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

Which means that the mole ratio of sulphuric acid : sodium carbonate is 1:1

Amount of sodium carbonate needed to neutralise 25cm³ sulphuric acid (average titre) = 24.90cm³

Moles Na2CO3 in 24.90cm³ of 0.100 mol/dm³ solution =

0.02490 x 0.1 = 0.00249 moles Na2CO3

Ratio Na2CO3 : H2SO4 = 1:1 therefore moles H2SO4 = 0.00249

0.00249 moles H2SO4 in 25cm³

Concentration of H2SO4 = 0.00249/0.025 = 0.0996 mol/dm³

Evaluation

The results and calculations show what I believe to be an accurate concentration of sulphuric acid that is 0.0996 mol/dm³. I carried out the experiment as stated in my method until I had concordant titres of within 0.1cm³ of each other. I did not include my rough titre in the average as it was performed to a much lower degree of accuracy and was merely to familiarise myself with the equipment and experiment.

I am almost certain that several errors have occurred in the experiment, mainly human error but also some caused by the procedure, technique and equipment. My reading of the meniscus in the burette could be inaccurate and therefore create an error. Even though I rinsed all my apparatus with distilled water before use (and the burette with sulphuric acid) it is still likely that some contamination occurred which would have affected my results. The amount of sulphuric acid I used in each titration was determined by when I saw a colour change in the indicator in the conical flask. It is very possible that I could have missed the exact moment when the colour change and therefore neutralisation occurred. This would mean too much or too little acid may have been added to the flask giving an inaccurate representation. If I were to repeat and develop this investigation in perfect conditions then I would try to devise a method that incorporated the use of a “light detector” which would give a second opinion of when a colour change had occurred to eliminate too much or too little acid being used.

The percentage errors associated with the experiment can be calculated by:

(Precision error x 100) / Actual reading

Errors caused by glassware and equipment

• Volumetric flask when filled correctly has a precision error of 0.2cm³ and therefore has a percentage error of 0.08%
• All burette readings include 2 decimal places in which the second figure was a 0 or a 5 as I could only determine between these measurements. This gives an error of 0.2% for each reading.
• The digital balance gave readings of 2 decimal places, which means the actual reading could be +/- 0.005g of the recorded reading; (0.005 x 100 /1.06 = 0.47) so the balance delivers a percentage error of 0.47%.

Errors caused by technique

• Mixing of the solution in the volumetric flask may not be totally through.
• The burette and pipette may not have been thoroughly washed out with the solutions used.
• The conical flask may not have been thoroughly washed out with distilled water between titrations.
• The end point may not be accurate if the solution from the burette is not added drop by drop with continuous swirling.
• Too much or too little indicator may have been added each time

It is not possible to place a value on the effect of human error on the reliability and accuracy of results. However, further repetition of the experiment would limit the effect human error has on results.

Improvements to the investigation would be mainly aiming to reduce the human error. This could be done by using equipment that displays values and measurements digitally, or detect the colour change more accurately for example using a light detector and beam of light through the conical flask. I could also use a balance of more than 2 decimal places.

Overall I do not believe my results could have been that inaccurate seeing as my titres were the same. I feel that the procedure allowed me to discover the accurate concentration of the acid to a fairly accurate and reliable degree.