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Module 6: Acid/Base Reactions

Quick questions on Buffer applications, blood pH, and Henderson-Hasselbalch explained: HSC Chemistry Module 6

13short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What is buffer composition and action (recap)?
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A buffer is a solution containing significant amounts of both a weak acid and its conjugate base (or a weak base and its conjugate acid). The two species sit in equilibrium:
What is the Henderson-Hasselbalch equation?
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Take logarithms of the $K_a$ expression:
What is the bicarbonate buffer in blood?
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Arterial blood is maintained at pH $7.40 \pm 0.05$ by a coupled buffer-respiratory-renal system. The dominant chemical buffer is the bicarbonate pair:
What is acid-base disturbances?
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In each case the body shifts the bicarbonate equilibrium to restore the 20:1 ratio.
What is response to added strong acid?
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The conjugate base consumes it:
What is response to added strong base?
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The weak acid consumes it:
What is step 2: Choose concentrations?
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Pick total buffer concentration ~0.20 mol/L for good capacity. With ratio 1.82:1, set $[HA] = 0.071$ mol/L and $[A^-] = 0.129$ mol/L (sum 0.20 mol/L).
What is calling any weak acid a buffer?
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A weak acid alone is not a buffer. A buffer needs both the weak acid and its conjugate base in comparable amounts.
What is forgetting the 20:1 ratio in blood?
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Most textbook buffers operate near 1:1 ($pH = pK_a$). Blood is held far from 1:1 because the components are open (lungs and kidneys), not closed.
What is confusing buffer capacity with buffer pH?
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Capacity is how much acid or base the buffer can absorb before pH shifts significantly; pH is the operating point. A dilute buffer has low capacity but the same pH as a concentrated one of the same ratio.
What is using strong acid in a buffer recipe?
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Henderson-Hasselbalch assumes the weak acid only partly ionises. A strong acid + strong base mixture is not a buffer.
What is misnaming acidosis and alkalosis?
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Acidosis is pH below 7.35 (more acidic); alkalosis is pH above 7.45 (more basic). Adding "respiratory" or "metabolic" specifies the origin.
What is ignoring the sign on log inside Henderson-Hasselbalch?
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$\log_{10}(ratio < 1)$ is negative, so pH is below $pK_a$ when the acid form dominates.

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