The aim of the experiment is to find the relative formula mass of an unknown acid in an acid solution that I was provided.

1.2 Introduction

I was provided with a solution of a monobasic (monoprotic) acid. Monoprotic acid is an acid that contains only one hydrogen atom. I was to determine the molarity of the acid by titration with a sodium hydroxide solution, and then use this molarity to calculate the relative formula mass of the acid. An acid is a substance that releases hydrogen ions when dissolved in water. There are two types of acids called organic acids and mineral acids. Organic acids, such as methanoic acid, citric acid or latic acid, are obtained from plant and animal sources. From the other side, mineral acids (hydrochloric acid, nitric acid or sulphuric acid) are made from minerals. They are much more reactive with bases than organic acids.

1.3 Equipments and apparatus

1. burette
2. 25 cm3 pipette
3. pipette filler
4. stand and clamp
5. funnel
6. 500 cm3 beaker
7. 250 cm3 conical flask
8. phenolphthalein indicator
9. wash bottle/deionised water
10. 250 cm3 of the unknown acid solution
11. 250 cm3 of 0.100 M sodium hydroxide solution
12. A sheet of paper to record the results of the titration
13. A white tile

1.4 Safety precautions

You must be careful throughout preparing the solution and the titration process. Some chemicals can pose a risk. In this case an alkali (sodium hydroxide) is corrosive chemical. In contact with skin it will cause burn and damage eyes. From the label of the unknown acid you can deduce that it is a toxic substance which is poisonous and can kill you. The care must be also taken while using phenolphthalein indicator. Its pH value is 9.5 which mean it is an alkali. This chemical also may cause damage to your body. During the experiment, if any of these chemicals affects part of your body immediately wash it with water. In order to avoid this, proper safety equipments should be used. You must wear fully buttoned lab coat, goggles and gloves.    

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2.1 Procedure

1. Take 100 cm3 of 0.100 M solution of sodium hydroxide
2. Prepare a solution with 1.5 g of an unknown acid up to 250 cm3
3. Wash and set up the burette. Fill it with the acid solution
4. Use the pipette to transfer 25.0 cm3 of the 0.100         M sodium hydroxide solution to a 250 cm3 conical flask.
5. Add 3-4 drops of phenol red indicator to the conical flask. Indicator will change its colour from pink in alkaline solution, to yellow in acid solution. When the solution first changes its colour to orange the endpoint is reached. The colour may fade after standing for a few minutes, but this can be ignored.
6. Add the acid from burette until the mixture just goes orange. Use the first titration to obtain an approximate value for the volume of the acid needed.
7. Repeat the titration twice more in order to obtain 3 concordant results. The results are concordant if they are within 0.10 cm3 of each other.
8. Record the results on a table

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2.3 Analysis

1. (a) Represent unknown acid as HA, and then write an equation for the reaction with sodium hydroxide.

(b) Calculate the molarity of a sodium hydroxide solution.

(c) The concentration of the acid solution was 1.5 g/250 cm3. Use this value to    calculate the relative formula mass of the acid

(d) Suggest a possible identity for acid HA which is known to be one of the following:

2. The acid solution was prepared by diluting a standard solution using a 250 cm3         volumetric flask. Assuming that the maximum errors for the apparatus used in the experiment were:

250 cm3 volumetric flask         ± 0.5 cm3

25 cm3 pipette                         ± 0.05 cm3

Burette                                     ± 0.15 cm3

            estimate the maximum percentage error in using each piece of apparatus, and the

            overall apparatus maximum error.

3. Compare your calculated Mr value with the Mr value of likeliest acid. Calculate

the percentage difference between these two values.

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3.1 Implementing

1. Wash all the equipments used in the experiment (including burette, conical flask,  

 pipette, pipette filler and beaker) with distilled water in the wash bottle.

2. Use pipette to transfer 25 cm3 of sodium hydroxide (alkali) solution into a conical flask.
3. Add 3 or 4 drops of phenol red indicator to the conical flask into which NaOH solution was put.
4. Fill the burette by pipette filler with acid solution up to point 0.00.
5. Take the point at which level of acid solution is least as the initial reading.

Wash the side of conical flask during the titration.

      6.   Swirl the solution in the conical flask for the indicator to disperse evenly and

continue swirling while adding the acid solution from the burette as well.

7. Start adding acid solution drop wise from the burette carefully to be able to

stop adding acid solution at a correct point when the endpoint is reached.

8. When the purple colour starts to change to yellow, stop adding acid solution. This is the endpoint.
9. Read correctly the final reading of volume on the burette when the endpoint is reached.
10. Repeat the titration two more times in order to obtain more accurate final results.

When repeating the titrations again uses the sodium hydroxide solutions in new conical flask and fills the burette again up to the point 0.00 since the amount of acid solution in one burette is not enough for all 3 titrations.

11. Record the results on a table

(Results are concordant, because they are within 0.10 cm3 each other.)

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3.2 Analysing

1. From the table above calculate the mean (average) amount of acid used to neutralise the NaOH solution.

 

      Calculation

 

The average amount of acid solution used = (25.9 cm³ + 26.0 cm³ + 26.1 cm³) / 3                        = 26.0 cm³

2. Equation of titration reaction where unknown acid is represented as HA.

HA (aq) + NaOH (aq) → NaA (aq) + H20 (l)

3. The number of moles of sodium hydroxide in the standard solution.

V (NaOH) = 25.0 cm³

M (NaOH) = 0.100 M

Calculation

Equation → n (NaOH) = (M × V) / 1000

                     n (NaOH) = (0.100 M × 25 cm³) / 1000 = 2.50 × 10−3 moles

4. The number of moles of the unknown acid

From the equation above (at the second step) we can see that the number of moles of sodium hydroxide solution equals the number of moles of the unknown acid. The ratio is 1:1.

n (HA) = n (NaOH)

This means:

n (HA) = 2.50 × 10−3 moles

5. The molarity of the unknown acid

              n (HA) = 2.50 × 10−3 moles

              V (HA) = 26.0 cm³

 

      Calculation

     

      Equation →   M= (n × 1000) ∕ V

                            M = (2.5 × 10−3 mol × 1000) ∕ 26 cm³ = 9.62 × 10−2 M

6. The relative formula mass (Mr) of the unknown acid

The mass of the unknown acid in 250 cm³ solution was 1.5 g.

n = (M × V) ∕ 1000       m (HA) = 1.5 g; M (HA) = 9.62 × 10−2 M;                         V (HA) = 250 cm³

n = m ∕ Mr

Combined equation

Mr = m ∕ n   → Mr = (m × 1000) ∕ (M ×V)

Mr = (1.5 g × 1000) ∕ (9.62 × 10−2 M × 250 cm³) = 62.37

7. The identification of the acid

From the table in the Analysis part by comparing the values of calculated Mr and fixed Mr, choose the likeliest acid.

The acid is nitric acid whose formula is HNO3

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4.1 Evaluation

While carrying out this experiment some errors occurred due to the value that was found.

1. The percentage maximum error in use of pipette

25 cm3 pipette              ± 0.05 cm3 (maximum error)

     

% error = (0.05 × 100) / 25 = ± 0.2%

2. The percentage maximum error in use of volumetric flask

250 cm3 volumetric flask       ± 0.5 cm3 (maximum error)

% error = (0.5 × 100) / 250 = ± 0.2%

3. The percentage maximum error in use of burette

Burette                        ± 0.15 cm3 (maximum error)

% error = (0.15 × 100) / 28 = ± 0.5%

4. The overall apparatus error

   

      The overall apparatus error = 0.2% + 0.2% + 0.5% = ± 0.9%

4.2 Comparison between experimental Mr with value of likeliest acid

My experimental value is 62.37 which are theoretically very close to the value of nitric acid from the table (shown in analysis part) whose value is 63. If I add the percentage of error to my experimental result, I will get the value of 63.27.

Final experimental result = experimental value + the overall apparatus error

                                                    = 62.37 + 0.9 = 63.27

% difference between experimental Mr and value of likeliest value    

= (62 / 63.27) × 100% = 100% − 97.99% = 2.01%

4.3 Conclusion

From the titration process I found the molarity of the sodium hydroxide solution and the relative formula mass of the unknown acid. With further calculations I identified the correct formula of the acid. The experiment was done without any major mistakes and complications, although there were some minor mistakes in calculations due to the apparatus used. This could be solved in another experiment by using more appropriate equipments. It will reduce the overall percentage error and minimize the mistake.