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Revision notes on chemical equilibrium - Explaining what a strong acid is and pH calculations - for Advanced A/AS Level Theoretical-Physical Chemistry
Doc Brown's Advanced A Level Chemistry Revision Notes
TheoreticalPhysical Advanced Level Chemistry Equilibria Chemical Equilibrium Revision Notes PART 5.3
5.3 Definition of a strong acid, theory, examples and pH calculations of strong acids
What is a strong acid? How to calculate the pH of a strong acid solution given its concentration.
5.3 Definition, examples and pH calculations of strong acids
Note: H+(aq) = aqueous hydrogen ion = aqueous proton = oxonium ion = hydroxonium ion
Appendix 1. What is the real aqueous hydrogen ion concentration in dilute sulfuric acid?
e.g. take 0.500 molar H2SO4 (aq), if fully ionised, you would expect ...
BUT, what is the reality?
The 1st dissociation is complete: H2SO4(aq) ==> H+(aq) + HSO4(aq)
The 2nd ionisation HSO4(aq) H+(aq) + SO42(aq) is not complete,
Prior to the 2nd ionisation, theoretically, the initial concentrations of hydrogensulfate ions and hydrogen ions will be equal, and both 0.500 mol dm3.
On the 2nd ionisation, the hydrogensulfate ion will provide the extra hydrogen ions and the only sulfate ions, but in doing so, the hydrogensulfate ion is reduced.
If we call the 'equal' extra hydrogen ion concentration and the final sulfate ion concentration x, then the total hydrogen ion concentration is (0.5 + x) and the hydrogensulfate ion concentration is reduced to (0.5 x)
This gives the quadratic equation 0 = x2 + 0.512x 0.006
On solving this using the quadratic equation formula, gives roots of 0.523 and +0.0114
Therefore x must be 0.0114 (the 'extra' H+), which then gives (0.5 + 0.0114) ...
a total hydrogen ion concentration [H+(aq)] of 0.511 mol dm3, not 1.000 mol dm3
and the real calculated pH = log10(0.511) = 0.29, not pH 0.00 and significantly higher
You may think the extra hydrogen ion concentration is very low, but this is because the HSO4 ion is a weak acid AND the 2nd ionisation is actually heavily suppressed by the 1st ionisation think Le Chatelier's equilibrium principle as regards the concentration effect.
To fully understand this calculation its handy to have studied the weak acid calculations page
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