Lesson 4 of 4

Titration curves and indicators.

About 25 minutes · 5 practice questions

A weak acid on the curve: a pH that climbs gradually, an equivalence point above 7 and the pKa half way. And how to choose an indicator that changes colour right on the steep part.

Step 1 of 5 · A weak acid

Step 1 of 5

A weak acid

Until now the flask held a strong acid. Here Acid in the flask is fixed at Ethanoic acid (weak), at the same concentration, 0.1 mol/L, and the same volume. The equivalence point is in the same place, 25 mL, because the number of moles is the same. Predict what will be different.

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Step 2 of 5

Half way

A weak acid’s curve has a flat stretch before the equivalence point. There the flask holds ethanoic acid and ethanoate, its ion, and such a mixture is called a buffer: it soaks up the $\mathrm{OH^-}$ ⁠ added, and the pH hardly moves. Look inside is now open at Whole curve.

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Step 3 of 5

The wrong indicator

An indicator is itself a weak acid whose acid and conjugate base have different colours. It changes colour around its own $\mathrm{p}K_a$ ⁠, over about two pH units. On the chart, the coloured band marks the current indicator’s range. Indicator is now fixed.

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Step 4 of 5

Choosing an indicator

The end point, where the indicator flips, should fall as close as it can to the equivalence point. Try the three indicators and move Volume added.

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Step 5 of 5

A weak base

Now the roles swap: the flask holds ammonia, a weak base, and the burette hydrochloric acid. The flask starts basic, and the pH falls as the acid goes in. Look at the curve and at the indicator’s band.

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Interactive scene

Loading the interactive version…

Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.

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Practice · Predict, then look

This time the flask holds ethanoic acid, a weak acid, at the same concentration and volume. What will the pH be at the equivalence point?

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Practice · Calculate

The equivalence volume is 25 mL. Change Volume added to 12.5 mL, half of it, and read the pH. What is the $\mathrm{p}K_a$ ⁠ of ethanoic acid?

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Practice · Predict, then look

Methyl orange changes from red to yellow between pH $3.1$ ⁠ and $4.4$ ⁠. If you titrate the ethanoic acid with it instead of phenolphthalein, when will the flask turn yellow?

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Practice · Choose

Which indicator suits titrating ethanoic acid with sodium hydroxide?

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Practice · Choose

Now the flask holds ammonia, a weak base, and the burette hydrochloric acid. The pH falls, and the equivalence point is at pH $5.3$ ⁠. Which indicator suits?