Nitric Acid – Highly Corrosive, toxic, oxidizing agent
Silver Nitrate – Poisonous, Corrosive to skin (Wear protective gloves and/or protective clothing)
1) First pipette approximately 10cm3 of each solution into separate boiling tubes and bubble carbon dioxide into each of the solutions.
The one goes milky (white precipitate) from colourless is the Calcium hydroxide so this can be eliminated. Label it.
2) The remaining 4 samples are still useable for the next test so, add 10cm3 of silver nitrate to all 4 of the solutions. One solution should give a cream precipitate (Bromide) and another should give a white precipitate (Chloride).
3) Separate the two samples and to these, add excess ammonia. The one which dissolves is silver chloride. Potassium Bromide and Sodium Chloride have now been eliminated. Label them.
4) The remaining solutions should be disgarded (the ones with silver nitrate in) and the remaining two solutions should be put in new boiling tubes (again, 10cm3). Now, to both solutions add copper (this should give a brown gas or blue solution.
Nitric Acid has now been eliminated.
5) Add 10cm3 of the only remaining solution to a new boiling tube. The last test is carried out just to make sure that the solution is what we think it is, ethanoic acid. Add iron (III) chloride solution should produce a red colour. Now label the boiling tube. Ethanoic acid had been eliminated.
Ca(OH)2 + CO2 –> CaCO3 + H2O
AgNO3 + NaCl –> AgCl + NaNO3
Add excess ammonia – Dissolves (more easily)
AgNO3 + KBr –> AgBr + KNO3
Add excess ammonia – Dissolves less easily
Cu + 4HNO3 –> Cu(NO3)2 + 2NO2 + 2H2O
Brown Gas or Blue solution
CH3COOH + FeCl3 –>
Part (ii) – Titration
Nitric Acid (Unknown concentration)
Aqueous Sodium Carbonate (0.500 mol dm-3)
Phenolphthalein (a few drops)
1) Set up the apparatus as shown so that the volumetric flask is sitting on the white tile and the burrette is pointing into the volumetric flask. Make sure that the burette is near eye level so that readings can easily be taken and so that they are accurate to 0.1cm3.
2) Pipette 25cm3 of Nitric acid (unknown concentration) into the volumetric flask and add a few drops of phenolphthalein indicator to the volumetric flask. The solution should remain colourless.
3) Fill the burette until it is on zero (or if slightly too much is added, the tap can be used to take it to zero).
4) Carefully and slowly open the tap and let the nitric acid enter the volumetric flask. The solution will turn pink where the nitric acid is dropping into to sodium carbonate but still keep adding the nitric acid. When the solution is nearly about to change, start adding the nitric acid drop by drop by closing the tap a until only a small amount gets through.
5) Once the solution has turned pink, take a note of the amount of nitric acid used (accurate to 0.1cm3.
N.B. Performing the titration more than once is always advised, so the experiment should be repeated at least twice and readings should be within 0.5cm3.
E.g. If 12.2cm3 of nitric acid was used and 25cm3 of sodium carbonate was used then this is how the concentration of the nitric acid would be calculated.
Na2CO3 + 2HNO3 –> 2NaNO3 + CO2 + H20
Moles Na2CO3 = (Volume x Concentration) / 1000
= (25 x 0.500) / 1000
= 0.0125 moles
0.0125 moles Na2CO3 –> 0.0250 moles HNO3
Moles HNO3 = (Volume x Concentration) / 1000
Concentration = (Moles / Volume) x 1000
= (0.0250 / 12.2) x 1000
= 2.05M (3sf)
Also you can read about Ka of ethanoic acid
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