CAIE A Level Chemistry 9701 · Equilibria

Equilibria: explain question

Explain7 marksChallenge

Human blood plasma contains dissolved $\ce{CO2}$ , hydrogencarbonate ions, $\ce{HCO3-}$ , and carbonic acid, $\ce{H2CO3}$ . The equilibrium below helps to maintain the pH of blood at about 7.4.

\ce{H2CO3(aq) <=> H+ (aq) + HCO3- (aq)}

(a) State what is meant by the term conjugate acid–base pair.

(b) Identify the conjugate acid–base pair in this equilibrium.

(c) Explain how this equilibrium acts as a buffer to resist a change in pH when a small amount of acid is added to the blood.

(d) Explain how this equilibrium acts as a buffer to resist a change in pH when a small amount of base is added to the blood.

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7 marks, so the mark scheme is looking for about 7 separate creditworthy points. Making the same point twice in different words earns it once.

Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.

Equilibria: explain question

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