Solution Y is known to contain chloride ions. You are to find the mass of silver chloride that can be obtained from 50.0 cm3 of solution Y. Measure the 50.0...
Question 1 Report
Solution Y is known to contain chloride ions. You are to find the mass of silver chloride that can be obtained from 50.0 cm3 of solution Y. Measure the 50.0 cm3 portion of solution Y into a conical flask and acidify it with dilute nitric acid. Add aqueous silver nitrate a little at a time, swirling the flask, until no more precipitate forms and then add 5 cm3 more. Separate the precipitate from the mixture using the apparatus shown in Fig. 1.1, wash it, dry it and find its mass. Enter every balance reading in Table 1.1 as you take it.
Weigh the filter paper before you begin and record its mass. Measure the portion of solution Y with a pipette, not a measuring cylinder, so that the volume is accurate. Swirl the conical flask after every addition of the aqueous silver nitrate. Keep adding until no more solid forms. Test for completeness by letting the solid settle and adding one more drop to the clear liquid above it. Filter the mixture. Wash the solid on the filter paper with distilled water. Dry the paper and the solid in a warm oven. Record the mass of the dry paper and solid.
Fig. 1.1
Table 1.1
| measurement | mass / g |
|---|---|
| filter paper before use | |
| filter paper and dry residue | |
| residue |
(a) Record your three balance readings in Table 1.1, giving each mass to the precision of the balance you are using. [3]
(b) Describe how you would use the apparatus in Fig. 1.1 to separate the precipitate from the mixture. [3]
(c) State why the residue is washed with distilled water before it is dried. [2]
(d) Explain why the residue is dried, weighed, dried again and weighed a second time. [2]
(e) Describe the table you would use to record the drying and weighing results. [2]
(f) Explain why the aqueous silver nitrate is added in excess. [2]
(g) Plan how you would adapt this method to compare the chloride content of two brands of table salt. [4]
(h) State two sources of error in this experiment. [2]
Answer Details
This is a gravimetric determination: chloride ions in solution Y are precipitated as silver chloride, \(\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl}\), which is filtered, washed, dried and weighed. The question tests accurate massing, filtration technique and evaluation.
(a) The three balance readings [3]. Record each mass to the balance's precision, for example two decimal places [1]; the mass of paper plus residue must be greater than the mass of the paper alone [1]; and the residue mass is worked out as mass of paper and residue minus mass of paper [1]. A representative set:
| measurement | mass / g |
|---|---|
| filter paper before use | 0.94 |
| filter paper and dry residue | 1.63 |
| residue | 0.69 |
(b) Separating the precipitate [3]. Fold the filter paper into a cone and fit it into the filter funnel, wetting it with distilled water [1]; stand the funnel in the neck of the conical flask [1]; pour the mixture down a stirring rod into the funnel so the precipitate is caught on the paper as the residue while the filtrate runs into the flask [1].
(c) Why wash with distilled water [2]. To wash away the excess silver nitrate and the other soluble salts left in the mixture [1]; otherwise these would dry on the residue and make its measured mass too large [1].
(d) Why dry, weigh, dry again and reweigh [2]. To check that the residue has reached constant mass [1]; when two successive weighings agree, all the water has evaporated and the mass recorded is that of the dry solid alone [1].
(e) The table for drying and weighing [2]. A table with a column for the drying number (or time in the oven) and a column for the mass in g [1]; one row per weighing, with headings and units given [1], for example:
| weighing number | mass of paper and residue / g |
|---|---|
| 1 | 1.65 |
| 2 | 1.63 |
| 3 | 1.63 |
(f) Why add silver nitrate in excess [2]. To make sure all of the chloride ions are precipitated [1]; if any chloride were left in solution the mass of the residue would be too small and the result too low [1].
(g) Adapting the method to compare two brands of salt [4].
- Weigh the same mass of each brand and dissolve each in the same volume of distilled water [1].
- Acidify each solution with dilute nitric acid and add the same excess of aqueous silver nitrate [1].
- Filter, wash and dry each precipitate to constant mass by the same method [1].
- Repeat each determination and compare the mean mass of residue from the two brands; the larger mean shows the higher chloride content [1].
(h) Two sources of error [2]. Any two: some precipitate is lost on the flask or the stirring rod; the residue is not completely dry when weighed; precipitate passes through a torn filter paper; the volume of solution Y is measured inaccurately [2].
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