Horvath Practice Problems 48

# Horvath Practice Problems 48 - we see that the H 3 O goes...

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12 WATER AND PH We learned about the pH scale last semester and we saw that it was a result of the fact that water is a weak electrolyte. That is water under goes autoionization, 2 H 2 O ( l ) ­ H 3 O + ( aq ) + OH ( aq ) (12.1) We can write the equilibrium constant for this reaction as K = K w = £ H 3 O + ¤£ OH ¤ . (12.2) Notice that we do not include the concentration of H 2 O because it a considered a pure liquid. At standard conditions of 25 C, K w =1 . 0 × 10 14 = £ H 3 O + ¤£ OH ¤ . By the symmetry of the reaction, two water molecules creates one H 3 O + and one OH ,thenforpurewater £ H 3 O + ¤ = £ OH ¤ =1 . 0 × 10 7 . If we now consider the addition of an Brønstad acid to water,
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Unformatted text preview: we see that the H 3 O + goes up: Q > K w . By LeChatelier’s principle, the autoionization is driven back-wards, H 3 O + ( aq ) + OH − ( aq ) → 2 H 2 O ( l ) , The only source of OH − is from the original water. Therefore the concentration of OH − goes down. More precisely the autoionization reaction is driven backwards until £ H 3 O + ¤£ OH − ¤ = K w . A similar situation arrises for addition of a Brønstad base. WATER AND PH 47...
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