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Acid-Base Equilibrium ­ Answers
1. Acids were first defined in terms of their sour taste. When indicators such as litmus were discovered, an acid was
thought of as any substance which turned litmus red.
2. Arrhenius theory of acids and bases:
Acids are substances which produce hydrogen ions in solution.…

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14. A weak acid is in equilibrium with its conjugate base in aqueous solution. The equilibrium constant is known in this
context as the acid dissociation constant Ka. The expression for this constant is:
[H O+][A-]
K a = 3 [HA] N.B. H2O is not included in this expression as…

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23. Sulfuric acid is only a strong acid in its first ionisation. Its second ionisation is weak:
HSO4- H+ + SO42-
The second ionisation is suppressed by the H+ ions from the first ionisation. So a solution of sulfuric acid of
concentration 0.10 mol dm-3 has [H+] of just above…

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The ammonium ions act as an acid, reacting reversibly with water to produce H3O+ ions, which make the solution
acidic. If you know the Ka value and conc. of the salt solution, you can then calculate the pH of the solution.
32. The anions in the salts of weak acids…

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The number of A- ions from the totally ionised salt is much greater than the few A- ions from the weak acid,
so it can be assumed that [A-] = [salt]
The ionisation of the weak acid is so suppressed that [HA] = [weak acid]
The equilibrium expression can, therefore,…

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Strong acid Strong base 1 7 3-11 Just < 13

Strong base Strong acid 13 7 11-3 Just > 1

Weak acid Strong base 3 9 7-11 Just < 13

Strong base Weak acid 13 9 11-7 Just > 3

Strong acid Weak base 1 5 3-7 Just < 11…

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Titration curve of acid added to alkali Titration curve of alkali added to acid
53. When a strong base is added to a solution of a weak acid, the point halfway to the equivalence point is when half
the acid has been neutralised. At the half neutralisation point, [HA] =…


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