20.3 the acid dissociation constant Ka + 20.4 the pH of weak acids

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9 Terms

1
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why are week acids week

they only partially dissociate

2
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Ka expression

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3
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what is pKa

  • a way of comparing numbers with negative indices

<ul><li><p>a way of comparing numbers with negative indices</p></li></ul><p></p>
4
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how does the strength of an acid affect the Ka value

  • strong acid = large Ka value

  • week acid = small Kc value

5
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how does the strength of an acid affect the pKa value

  • strong acid = small pKa value

  • weak acid = large pKa value

6
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2 approximation which simplify the Ka equation in weak acids

  • the H+ ions from the dissociation of water are very small and can be neglected

  • neglect and decrease in the concentration of HA from dissociation

7
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simplified Kexpression

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8
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how to calculation [H+] of a week acid given its concentration + Ka

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9
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when will the approximations needed to calculate weak acids begin to break down

  • the first approximation assumes that the dissociation of water is negligible which will break down for very week acids/ dilute solutions

  • the second approximation assumes that the concentration of acid is much greater than the H+ concentration at equilibrium. this is not justified for stronger weak acids