Sulfuric acid, H2SO4, readily dissociates into H+ and

Chapter 5, Problem 87P

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Problem 87P

Sulfuric acid, H2SO4, readily dissociates into H+ and HSO−4 ions:

The hydrogen sulfate ion, in turn, can dissociate again:

The equilibrium constants for these reactions, in aqueous solutions at 298 K, are approximately 102 and 10−19 respectively. (For dissociation of acids it is usually more convenient to look up K than ΔGo, By the way, the negative base-10 logarithm of K for such a reaction is called pK, in analogy to pH. So for the first reaction pK = −2, while for the second reaction pK = 1.9.)

a) Argue that the first reaction tends so strongly to the right that we might as well consider it. to have gone to completion, in any solution that could possibly be considered dilute. At what pH values would a significant fraction of the sulfuric acid not be dissociated?

b) In industrialized regions whore lots of coal is burned, the concentration of sulfate in rainwater is typically 5 × 10−5 mol/kg. The sulfate can take any of the chemical forms mentioned above. Show that, at this concentration, the second reaction will also have gone essentially to completion, so all the sulfate is in the form of . What is the pH of this rainwater?

c) Explain why you can neglect dissociation of water into H+ and OH− in answering the previous question.

d) At what pH would dissolved sulfate be equally distributed between  and ?