MC test: pH curves

Multiple choice test on 18.3 pH curves

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

When 49.0 cm3 of 1.0 mol dm−3 sodium hydroxide solution is added to 50.0 cm3 of 1.0 mol dm−3 hydrochloric acid solution the pH of the resulting mixture is 2. What will be the pH of the resulting mixture when 49.9 cm3 of 1.0 mol dm−3 sodium hydroxide solution is added to 50.0 cm3 of 1.0 mol dm−3 hydrochloric acid solution?

There will be 0.1 cm3 remaining unreacted of the 1.0 mol dm−3 hydrochloric acid solution in 99.9 (≈ 100) cm3 of solution. [H+(aq)] = 1 x 10−3 mol dm−3 and the pH = 3.

 

A graph was obtained using the data from a titration.

Which titration provided the data for the graph?

The curve is the typical shape for a weak base titrated with a strong acid.

 

Which statements are correct concerning titration curves?

I. The equivalence point depends upon the indicator used.

II. The pH at the half-equivalence point of a weak acid - strong base titration is equal to the pKa value of the weak acid.

III. There is no sharp change in pH around the equivalence point in a weak acid - weak base titration.

The equivalence point depends upon the stoichiometry of the reaction. The end-point (where a permanent colour change occurs) depends upon the precise indicator used.

 

It requires 25.0 cm3 of a 0.080 mol dm−3 solution of sodium hydroxide to reach the equivalence point when it is titrated with 20.0 cm3 of 0.100 mol dm−3 ethanoic acid solution. (pKa of ethanoic acid = 4.8)

What volume of the sodium hydroxide solution needs to be added to 20.0 cm3 of the ethanoic acid solution to give a mixture with a pH of 4.8?

The pH at the half-equivalence point will be equal to the pKa value of the acid.

 

Some information about indicators is given.

Indicator pKa pH range colour in acid colour in alkali
bromophenol blue 4.2 3.0 - 4.6 yellow blue
methyl red 5.1 4.4 - 6.2 red yellow
bromothymol blue 7.0 6.0 - 7.6 yellow blue
phenolphthalein 9.6 8.3 - 10.0 colourless pink

Which indicator should be chosen when ethanoic acid is titrated with potassium hydroxide solution?

For a weak acid-strong base titration most of the rapid change in pH will occur in the pH region of 8 - 10 so only phenolphthalein should be used.

 

An acidic indicator can be represented by HIn(aq) and a basic indicator can be represented by B(aq).

HIn(aq) (colour A) ⇄ H+(aq) + In−(aq) (colour C)

B(aq) (colour B) + H2O(l) ⇄ OH−(aq) + B+(aq) (colour D)

Which row gives the correct colours for the indicators in acidic solution?

Row Acidic indicator Basic indicator
1 A B
2 A D
3 C B
4 C D

In acidic solution the position of equilibrium will lie on the reactants side for the acidic indicator and on the products side for the basic indicator.

 

Which salt will dissolve in water to give a solution with a pH > 7?

Sodium carbonate is a salt formed by reacting a weak acid with a strong base so will dissolve in water to give an alkaline solution. Sodium nitrate and sodium chloride will give neutral solutions as they are the salts of strong acids and strong bases whereas ammonium chloride will be acidic as it is the salt of a weak base and a strong acid.

 

The titration graph is for a weak acid titrated with a strong base.

Which letter denotes the buffer region?

The central point of the buffer region is the half-equivalence point as there is very little change in pH when small amounts of alkali or acid are added.

 

Which mixture produces a buffer solution?

When excess ethanoic acid is added to sodium hydroxide solution the mixture will contain sodium ethanoate and ethanoic acid, i.e. the salt formed from a weak acid and a strong base together with the weak acid which gives an acidic buffer.

 

A student sketched the following graph to show the change in pH during the titration of a strong monoprotic acid with aqueous sodium hydroxide solution .

Which statements can be correctly deduced?

I. The initial concentration of the monoprotic acid was 0.1 mol dm3.

II. The concentration of the sodium hydroxide solution was 0.1 mol dm3.

III. The shape of the graph is incorrect between pH values of 5 to 9.

Initially the pH is 1 so [H+(aq)] = 0.1 mol dm−3. The equivalence point is when 25.0 cm3 of NaOH has been added but the initial volume of the acid is not given so the concentration of the NaOH cannot be deduced. For a strong acid - strong base titration there is a rapid change in pH at the equivalence point so the shape of the graph between pH 5 and 9 should be almost vertical.

 

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