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Module 5 Chapter 20 (20.1 Bronsted-Lowry acids and bases (Ionic equations,…
Module 5 Chapter 20
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20.2 pH scale
pH and [H+]
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i.e. to dilute a solution from pH 1 to pH 4 (3 units) would require dilution by 10x10x10= 1000 times
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20.5 pH and strong bases
Ionisation of water
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Neutral can be at different pH's for different temperatures so even if pH doesn't equal 7 if [H+]=[OH-] it's neutral
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20.4 pH of weak acids
[H+ (aq)] depends on
1) Concentration of acid, [HA(aq)]
2) Acid dissociation constant, Ka
Approximations
Number 1
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Limit- very small amount of H+ produced from water- ignored as its negligible compared to [HA (aq)], after pH 6 this becomes significant however
Number 2
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Limit- small amounts of HA dissociate no matter how weak the acid is so not equal. Weak acids with small Ka fit second part but if Ka > x10-2 [H+] becomes more significant
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With approximations Ka=[H+ (aq)]^2 / [HA(aq)]
Must still write out full equation before simplifying!!
Can determine Ka by preparing a standard solution of known conc. and measuring pH with a pH meter, then use equation