analyticalChemistryToughJob
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helpfulmaterials.zipassignhints(1).pptx
Hints for the Assignment Weak Monoprotic Acids
| Name | Formula | Value of Ka |
| Monochloracetic acid Hydrofluoric acid Nitrous acid Formic acid Lactic acid Benzoic acid Acetic acid Hydrated aluminum(III) ion Propanoic acid Hypochlorous acid Hypobromous acid Hvdrocyanic aid Boric acid Ammonium ion Phenol | HC2H3ClO2 HF HNO2 HCO2H HC3H5O3 HC7H502 HC2H3O2 [Al (H20)6]3+ HC3H5O2 HOCl HOBr HCN H3BO3 NH4+ HOC6H5 HOI | 1.35 x 10 -3 7.2 x 10-4 4.0 x 10-4 1.8 x 10-4 1.38 x 10-4 6.4 x 10-5 1.8 x 10-5 1.4 x 10-5 1.3 x 10-5 3.5 x 10-8 2 x 10-9 6.2 x 10-10 5.8 x 10-10 5.6 x 10-10 1.6 x 10-10 2 x 10-11 |
Weak Alkalis or Bases
| Formula | Name | Kb | |
| NH3 | ammonia | 1.8 x 10 -5 | |
| CH3NH2 | methylamine | 4.4 x 10 -4 | |
| C5H5N | pyridine | 1.5 x 10 -9 | |
| C6H5NH2 | aniline | 4.3 x 10 -10 | |
It is possible to set up a simulated titration between a weak acid and a strong base.
It is easier to work backwards from pH to the added volume of titrant using the equations below as [H+] = 10(-pH)
It is possible to generate the curve with just one formula going backwards.
However we know by working forwards from added volume to pH that there are certain key points of interest, namely where φ = 0, 0.5, 1 and >1.
Solver in excel
Looking at the phi we have slightly missed the key point working backwards.
Using solver excel can vary the pH until the equations give a target value.
The blue cell is the set objective.
Excel will change the value in cell A58 until the objective cell reads 0.5. To speed things up we constrain the trial values to lie between 4.75 and 4.8.
Hence we can tweak the pH values to give the key phi values as these may be of academic interest.
The formulae for titrating a weak alkali with a strong base are similar to the above.
Some sources use more difficult notation.
The treatment is for monoprotic acid (those that release at most one hydrogen ion per formula unit).
Polyprotic acids have different dissociation constants for the first, second etc. hydrogen ions and hence produce a titration curve with many steps.
There are relatively few common bases and no issues corresponding to the polyprotic complexity.
