25/100 = 0.25 mol dm-³
This low concentration is used because it is safer to use, a highly concentrated solution is too corrosive and it would be hard to see when the solution with the indicator changes colour as the reaction will happen to fast as a large number of moles comes from each drop of acid. A low concentration would make the indicator have a too great of an effect on the end result.
Distilled water is used to wash out the conical flask and funnel, and the sodium carbonate solution (0.100 mol dm-³ ) is used to wash the pipette . The sulphuric acid is used to wash the burette.
25 cm³ of the aqueous solution of sodium hydroxide is put into a conical flask using a pipette and filler. The amount in the pipette is seen from the line on the pipette which shows 25 cm³ of solution. For accuracy, the amount should be looked from under the line.
The hydrochloric acid is put into the burette using the funnel. It is measured to approximately 0.00 cm³ but does not have to be exactly this. The number is recorded. The burette is held by a clamp and stand
A white tile is put under the burette and the conical flask is placed on the white tile. The tip of the burette should be in the conical flask but not touching the liquid.
Methyl Orange indicator is added to the conical flask (about 4 to 5 drops).
The acid is then added to the conical flask while the conical flask is shaken slightly. This is done until the solution in the conical flask turns pink.
The volume on the burette is now recorded. This volume minus the starting volume gives you the volume used in the experiment. This procedure is repeated until there are three results within 0.1 cm³ of each other. The average of these results are taken and is used in the final calculation.
Diagram
Reaction
H SO + NaOH NaSO + H O
Calculation to find the concentration
Moles of sodium hydroxide are worked out first using:
Moles = concentration x Volume
Than the ratio of moles is worked out to find moles of sulphuric acid
1:1
So moles of sulphuric acid are the same as moles of sodium hydroxide.
To work out concentration
Concentration = Moles
Volume
Method 2—Using Gas collection to determine the concentration of sulphuric acid.
I am going to determine the concentration of sulphuric acid by finding out how much gas will be produced from the reaction between magnesium carbonate and a certain volume of sulphuric acid. The concentration of sulphuric acid given is 10 mol dm-³ so I will need to dilute this down. Gas will be collected in a burette and when no more gas is produced, the volume in the burette can be used to work out the concentration of the sulphuric acid.
Safety
Safety goggles will be worn to protect eyes from any splashes of irritants. Gloves will be used to protect hands from any contact with corrosive, irritant or toxic substances.
Equipment
1g Magnesium carbonate
Sulphuric acid ( 10 mol dm-³ sulphuric acid.)
Distilled water
Divided flask
Rubber delivery tube
Pipette.
Trough
Temperature controlled bath
Burette
Diagram
How much volume of sulphuric acid should be used
If too much volume of the sulphuric acid is used then the volume of gas will be larger than the volume of the burette which would make the experiment a failure. I have found out that if the sulphuric acid was diluted to approximately 0.25 mol dm-³ than the reaction with the magnesium carbonate would give 60 cm³ of carbon dioxide if 10 cm³ of acid was used. This amount of gas would fit in the burette.
i.e.
Moles sulphuric acid = Concentration x Volume = 0.25 x 0.01 = 0.0025 M
Moles ratio between sulphuric acid and carbon dioxide = 1:1
So moles of carbon dioxide = Moles x 24000 = 0.0025 x 24000 = 60 cm³
Procedure
The sulphuric acid is diluted to 0.25 mol dm-³ by putting 25 cm³ in a volumetric flask and then putting 100 cm³ of distilled water in. This will equal:
25/100 = 0.25 mol dm-³
Remember that all the equipment should be placed in a temperature-controlled bath so that the temperature will not affect the gas produced as a high temperature would make the gas expand increasing the volume.
The divided flask is washed out with distilled water. The reactants are put in a dividing flask so therefore they are separated. Excess magnesium carbonate should be added to make sure all of the acid reacts.
If 0.25g was used then. Moles Magnisium carbonate = 0.25
84.3
=0.003M
Moles ratio 1:1
Moles carbon dioxide = 0.003M
Volume = 0.003 x 24000 = 72 cm³
So 0.25g is a good amount even if magnesium carbonate was used in excess.
A bung is put on top of the dividing flask after a rubber delivery tube is placed inside the divided flask.
A burette is held by a clamp and stand with the burette placed upside down in the trough . A trough is filled with distilled water until the distilled water reaches almost 0.00 cm³ on the burette. This volume on the burette is recorded
The end of the rubber delivery tube is placed inside the burette.
The reactants are now allowed to mix.
When the gas production has stopped than the volume of gas in the burette can now be found out. This is done by recording the volume of water now and minusing this volume from the volume of water recorded at the start of the experiment.
Repeat until 3 results are 0.1cm³ within each other.
Reaction
H SO + MgCO MgSO + H O + CO
Calculation to work out the concentrations.
Moles = Volume is to work out the moles of the carbon dioxide
24000
Than to work out the moles of sulphuric acid the ratio of moles of sulphuric acid and carbon dioxide is worked out
1:1
So to work out the concentration
Concentration = Moles
Volume
Calculation from specimen titration experiment
Reference
Adams grammar A level chemistry experimental skills booklet– page 4
The website http://pubs.acs.org/cgi-bin/abstract.cgi/iecac0/1942/14/i07/f-pdf/f_i560107a025.pdf?sessid=2640
