Titration techniques; volumetric analysis & stoichiometry：練習問題

Which of the following procedures is MOST likely to introduce a systematic error in an acid-base titration?

In a titration experiment, 20.0 cm³ of a 0.150 M calcium hydroxide solution was neutralised by 30.0 cm³ of a nitric acid solution. What is the concentration of the nitric acid?

A 1.000 g solid sample containing a mixture of anhydrous sodium carbonate (\(\text{Na}_2\text{CO}_3\)) and sodium bicarbonate (\(\text{NaHCO}_3\)) is dissolved in water, and the resulting solution is accurately made up to \(250.0 \text{ cm}^3\).
A \(25.00 \text{ cm}^3\) aliquot of this solution is titrated with \(0.1000 \text{ M}\) hydrochloric acid (\(\text{HCl}\)).

When phenolphthalein is used as the indicator, the titre volume required is \(6.00 \text{ cm}^3\) (\(V_1\)).
In a separate titration of a \(25.00 \text{ cm}^3\) aliquot using methyl orange as the indicator, the total titre volume required is \(16.34 \text{ cm}^3\) (\(V_2\)).

Calculate the percentage by mass of sodium carbonate (\(\text{Na}_2\text{CO}_3\)) in the original solid sample.

(Molar masses: \(\text{Na}_2\text{CO}_3 = 106.0 \text{ g mol}^{-1}\), \(\text{NaHCO}_3 = 84.0 \text{ g mol}^{-1}\))

In a titration between a weak organic acid and sodium hydroxide, a student finds that the endpoint is not sharp. Which of the following modifications is LEAST likely to improve the precision of the result?

In a titration experiment, 10.0 cm³ of a 0.500 M barium hydroxide solution was neutralised by 40.0 cm³ of a nitric acid solution. What is the concentration of the nitric acid?

State TWO reasons why a conical flask used in an acid-alkali titration should be rinsed with distilled water but NOT with the solution it will contain.

In a titration setup, WHY is it necessary to ensure there are no air bubbles trapped in the tip of the burette before starting the experiment?

A student performed a titration to determine the concentration of an unknown hydrochloric acid solution. They used a standard solution of \(0.125 \text{ mol dm}^{-3}\) sodium carbonate (\(\text{Na}_2\text{CO}_3\)).

(a) State one property that makes anhydrous sodium carbonate a suitable primary standard for volumetric analysis.

(b) During the titration, \(25.00 \text{ cm}^3\) of the standard sodium carbonate solution required an average of \(20.80 \text{ cm}^3\) of the hydrochloric acid solution for complete neutralisation. The balanced chemical equation for the reaction is:
\(\text{Na}_2\text{CO}_3(\text{aq}) + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})\)
Calculate the concentration of the hydrochloric acid solution.

(c) Suggest a suitable indicator for this titration and state the expected colour change observed at the equivalence point.

A student carried out an experiment to determine the purity of a sample of ammonium sulfate, \((NH_4)_2SO_4\). A \(1.50 \text{ g}\) sample of the impure salt was heated with an excess of sodium hydroxide solution. The ammonia gas evolved was completely absorbed into a flask containing \(50.0 \text{ cm}^3\) of \(0.250 \text{ mol dm}^{-3}\) sulfuric acid, \(H_2SO_4\).

After the reaction was complete, the excess sulfuric acid in the flask required \(25.60 \text{ cm}^3\) of \(0.200 \text{ mol dm}^{-3}\) sodium hydroxide solution for complete neutralization.

(a) Write a balanced chemical equation for the reaction between ammonium sulfate and sodium hydroxide upon heating.
(b) Calculate the number of moles of \(H_2SO_4\) initially added to the absorption flask.
(c) Calculate the number of moles of \(NH_3\) that were evolved from the sample.
(d) Determine the percentage purity by mass of ammonium sulfate in the original sample.
(Relative atomic masses: \(H = 1.0, N = 14.0, O = 16.0, S = 32.1\))