Determining the Concentration of Sulphuric acid

Salters as Practical Assessment-Plan

I am going to plan an experiment to determine the accurate concentration of sulphuric acid. It is thought to have the concentration between 0.05 and 0.15 moldm-3. I will be provided with a solid base which is anhydrous (powder) sodium carbonate (Na2CO3). To find the accurate concentration I will titrate the sulphuric acid against the sodium carbonate. The reaction following will take place:

Na2CO3 (aq) + H2SO4 (aq) Na 2 SO4 (aq) + CO2 (g) + H2O(l)1

Deciding the Amounts

Sulphuric acid has a concentration about 0.10 moldm-3 (half way between 0.05 and 0.15 moldm-3). I would like to use 25cm3 of sulphuric acid. This is because, it is not a wasteful amount and also it would reduce percentage errors because this is quite a large amount to use. It would be an ideal value to use.

Furthermore, I will need to make up a standard solution from the solid base that I have been provided with, which is sodium carbonate. I would again ideally like to use 25cm3 of the standard solution per titre. This is because, the pipettes have the reading of 25cm3, which means the pipette is readily, available for this amount, therefore it is a sensible value.

Consequently I will need to make up a standard solution of concentration 0.10moldm-3. This is because of the stoichiometry. The equation shows that for every one mole of sulphuric acid, there is one mole of sodium carbonate. Therefore, the concentration must be the same because the volumes are the same and the number of moles are the same. I estimated the concentration of sulphuric acid to be 0.10 moldm-3 because it’s halfway between the thought amount of 0.05 and 0.15 moldm-3, therefore the concentration of the standard solution must be the same.

Deciding what Indicator to Use

I need to use an indicator to see when the end point of the titration occurs, and they are used between acid and alkalis. I have decided on using 2-3 drops of methyl orange indicator fin my experiment per titre. This is because; methyl orange indicator gives an accurate indication of the end point using a strong acid (sulphuric acid) and a weak alkali (sodium carbonate). 2

Making up my Standard Solution

As I have mentioned before, I want my standard solution to have the concentration of 0.10 moldm-3. I have chosen my volumetric flask to be 250cm3. This is because these volumetric flasks are readily available. In addition, I am aiming to do approximately five titrations, therefore this amount would allow me to do five titrations and have some to spare.

I now need to find out how much sodium carbonate (Na2CO3) is needed to make a standard solution of concentration 0.10 moldm-3.

I need to use the equation:

Number of moles = concentration (moldm-3) x volume(dm3)3

n = c x v

The volume is 250cm3 so I need to divide this by 1000 so that it converts into dm3.

Number of moles = 0.10(moldm-3) x 0.25 (dm3)

Number of moles = ...

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I now need to find out how much sodium carbonate (Na2CO3) is needed to make a standard solution of concentration 0.10 moldm-3.

I need to use the equation:

Number of moles = concentration (moldm-3) x volume(dm3)3

n = c x v

The volume is 250cm3 so I need to divide this by 1000 so that it converts into dm3.

Number of moles = 0.10(moldm-3) x 0.25 (dm3)

Number of moles = 0.025 moles

Therefore, 0.025 moles id required for the standard solution, nevertheless, I now need to convert this into grams by using the equation:

Number of moles = Mass (g) / Molar mass(gmol-1)4

Mass (g) = Number of moles x Molar mass(gmol-1)

Molar mass for Na2CO3 = (23 x 2) + 12 + (16 x 3) = 106gmol-1

Mass (g) = 0.025 x 106 = 2.65g

Therefore, 2.650g of sodium carbonate is required for my standard solution.

My experiment

The apparatus I am going to use are as follows:

Burette
Conical flask
Volumetric flask
Rubber ball pipette filler
Distilled water
Beakers
Filter funnel
Teat pipette
Tile
Sulphuric acid
Sodium Carbonate
Scales
Spatula
Weighing bottle
Glass rod
Stand
Clamp
Methyl orange indicator

Plan of Experiment

Prepare apparatus.
Weigh a weighing bottle accurately. Using a spatula, transfer 2.650g of sodium carbonate to the weighing bottle and weigh again.
Tip the contents of the weighing bottle into a clean 250cm3 beaker and reweigh the “empty” weighing bottle to find the weight by difference. This way I can work out the actual weight of sodium carbonate that has been transferred.
Add 50cm3 of distilled water to the solid and stir using a glass rod until dissolved. Transfer this solution into a volumetric flask using a funnel. Make up the graduation mark with distilled water using a teat pipette to ensure you so not overshoot it. This is now the standard solution.
Put a stopper on the flask and invert it about 20 times to make sure all the solution is mixed.
Using pipette filler, pipette 25.0cm3 of the solution into a clean conical flask.
Clean the burette out with distilled water and then again with sulphuric acid. Attach the burette to the stand, and fill the burette using a funnel with sulphuric acid.
Add methyl orange indicator to the conical flash only about 2-3 drops. Put the conical flask onto a white tile so the colour change is easy to see when titrating.
Titrate the solution until you reach the end point (when the solution turns orange), and record your results. I am going to use this as a rough titre.
Repeat this to obtain concordant results, concordant meaning results that are similar usually results which are in 0.1cm3 of each other.

Risk Assessment

Why my plan will Devise Reliable results

My plan will devise reliable result because of many reasons. Firstly I am going to do a rough titre in my plan, which means I will have a rough idea of my titration, so when I do my real results I know when to add it drop wise, so that I won’t overshoot the end point. Another reason why I won’t overshoot the end point is because I am going to use a white tile so I can judge the end point more accurately.

Furthermore, I am only going to add 2 or 3 drops of methyl orange indicator. This will make my results more reliable because the methyl orange indicator is slightly acidic, so it would use up the alkaline solution, therefore by only adding a small amount of it will make my results more reliable and accurate.

In addition, I am also going to wash my equipment out thoroughly with distilled water and then with the solution I am going to put that piece of equipment. This will remove any impurities that were originally in the containers so will make my results more reliable.

Also, when I am measuring the solution, I will measure accurately by using a pipette to make sure the bottom of the meniscus is on the line to make my results more accurate. Also, when reading the burette I will put a piece of plain paper behind so I can read it more accurately.

I will also, remember to take the funnel out of my burette as this could also affect the accuracy of my results. This is because drops are still on the funnel could enter my burette which mean more sulphuric acid affecting my results. This way by removing the funnel, I will achieve accurate results. 7

Finally, I am going to use accurate equipment as possible, which will minimise errors in my results.

Overall my results will be very accurate and reliable!

References

1. Article on types of chemical reactions, . I used this to help me write my balanced equation for the reaction between sodium carbonate and sulphuric acid.

2. Information on the use of indicators in acid-alkali titrations, salters practical assessment. I used this to decide what indicator to use as it told me what to use if I had a strong acid and weak alkali.

3. Chemical ideas, 2nd edition, chapter 1, section 1.5, concentrations of solutions, published by Heinemann 2000. I used this to get the concentration equation.

4. Power point presentation on moles and concentration, . This helped me find the formulas for concentration and to find out the mass.

5. Safety data sheet, . I used this source to find out the safety statement of sodium carbonate and the precautions I will need to take.

6. Hazard cards, numbers 98 and 32, Cleapps 1998 and 2003. I used these to find out the safety statement and also the precaution I will need to take when doing my experiment for methyl orange indicator and sulphuric acid.

7. Skills for Salters as Chemistry, pages 3,4,6,7 and 29, chemistry department, 2007. I used this to see how I will use my equipment and use it accurately, giving me accurate results.

Analysis

I made up my standard solution by firstly weighing out some sodium carbonate.

I weighed the weighing bottle and added approximately 2.65g of sodium carbonate. I then transferred the sodium carbonate to a beaker and reweighed the weighing bottle, so I can therefore work out exactly how much sodium carbonate I transferred.

Mass of weighing bottle and sodium carbonate = 57.732g

Mass of weighing bottle after transfer of sodium carbonate = 55.052g

Consequently the accurate amount of sodium carbonate transferred is the mass of the weighing bottle and the sodium carbonate, minus the mass of the weighing bottle after the transfer which is:

57.932g – 55.052g = 2.680g

Therefore, 2.680g of sodium carbonate was transferred.

Next I added about 50cm3 of water to the beaker and using a glass rod I stirred the solution until the sodium carbonate has dissolved. This was then fully transferred to a volumetric flask and water was added to the 250cm3 graduation mark. I then used a pipette to transfer 25.00cm3 of this standard solution to a conical flask and titrated with the unknown acid.

I used 25.00cm3 of the standard solution for each titre after the experiment; I had obtained results for how much acid was added for each titre. The results for how much acid I added for each titre are as follows: 20.10cm3, 19.85cm3 and 20.05cm3. I am now going to work out an average by picking out the concordant results (results which are in 0.10cm3 of each other) and dividing by how many there is.

20.10cm3+ 20.05cm3/2 = 20.075cm3.

Therefore, my average titre was 20.075cm3.

Concentration of Sodium Carbonate Solution

I am now going to calculate the concentration of the sodium carbonate solution:

2.68g of sodium carbonate was dissolved in a 250cm3 volumetric flask using water, so I need to find out how many moles of the substance I used in order to find out the concentration. Therefore I need to use the equation:

Number of Moles= Mass (g)/Molar Mass (gmol-1)

Molar mass of Na2CO3 = (23 x 2) + 12 + (16 x 3)= 106gmol-1

Number of Moles= 2.680g/ 106gmol-1

Number of Moles= 0.0253moles.

Therefore, I used 0.0253 moles in my standard solution.

I now need to use the equation to find the concentration:

Concentration (moldm-3) = Number of moles/Volume (dm3)

I firstly need to convert 250cm3 to dm3 so that my equation works:

1000cm3 = 1dm3

250cm3 = 0.250dm3 (I divided by 1000).

Concentration (moldm-3) = 0.0253moles/0.250dm3

Concentration= 0.101moldm-3

Consequently, I have calculated my sodium carbonate standard solution to have a concentration of 0.101moldm-3, rounded to three significant figures.

Concentration of acid

I am now going to calculate the concentration of the acid solution; therefore I firstly need to find out how many moles there is of the sodium carbonate solution, by using the equation:

Concentration of Na2CO3 (moldm-3) x Volume of Na2CO3(dm3) = Number of moles of Na2CO3

0.101moldm-3 x (25/1000) dm3) =0.00253 moles

The equation of my reaction is:

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

This equation shows that the molar ratio of sodium carbonate and sulphuric acid is 1:1 which means is there is 0.00253 moles of sodium carbonate there is 0.00253 moles in sulphuric acid.

The average titre as shown above was 20.075cm3, which I need to convert to dm3 which is:

20.075cm3/1000 = 0.0201dm3.

I can now use the equation

Concentration (moldm-3)= Number of moles/Volume (dm3)

Concentration (moldm-3)= 0.0253 / 0.0201

Concentration = 0.126moldm-3.

Overall, I conclude that the concentration of the acid solution is 0.126moldm-3.