Qualitative Analysis (Practical) Flashcards
Describe the solubility of the following ions.
1: NO3 ^-
2: SO4 ^2-
3: CO3 ^2-
4: O ^2-
5: OH ^-
6: PO4 ^3-
7: Cl ^-
8: Br ^-
9: I ^-
10: sodium salts
11: potassium salts
12: ammonium salts
1: All nitrates are soluble.
2: All sulfates are soluble except lead sulfate (Pb ^2+), barium sulfate (Ba ^2+), and calcium sulfate (Ca ^2+).
3: All carbonates are insoluble except Na+, K+ and NH4+.
4: All oxides are insoluble except Na+ and K+.
5: All hydroxides are insoluble except Na+ and K+. Ba ^2+ and Ca ^2+ are slightly soluble.
6: All phosphates are insoluble except Na+, K+ and NH4+.
7: All chlorides are soluble except Ag+ and Pb 2+.
8: All bromides are soluble except Ag+ and Pb 2+.
9: All iodides are soluble except Ag+ and Pb 2+.
10: All sodium salts are soluble.
11: All potassium salts are soluble.
12: All ammonium salts are soluble.
Name the 2 main reagents used to test for cations.
Aqueous sodium hydroxide and aqueous ammonia
Describe how the reagents NaOH and NH3 should be added, and the factors to take note of if precipitate forms.
The reagents should be added drop-wise, then in excess. If precipitate forms, the colour of precipitate must be noted and the solubility of the precipitate in excess alkali must be checked.
Explain if solid sodium hydroxide pellets would serve the same purpose as aqueous sodium hydroxide?
No. If solid solution was used, the NaOH pellets may cause difficulty in observing the formation of precipitate(s).
Which hydroxides react with aqueous sodium hydroxide for precipitation to occur?
All metal hydroxides except for Group 1 hydroxides.
What type of hydroxides dissolve in excess NaOH (aq) due to formation of complex ions? Give 3 examples.
Amphoteric oxides.
Zn2+, Al3+, Pb2+
When NaOH (aq) is added to Pb2+ (aq) till excess, predict what will happen. Give the full process.
Then, write the ionic equation for each reaction formed respectively.
A white precipitate Pb(OH)2 is first formed {1} which dissolves due to formation of complex ion [Pb(OH)4] ^2-. {2}
Reaction 1: Pb2+ (aq) + 2OH- (aq) -> Pb(OH)2 (s)
Reaction 2: Pb(OH)2 (s) + 2OH- (aq) -> <- [Pb(OH)4] ^2- (aq)
*Reaction 2 is reversible, thus the double arrows
State the ion(s) being identified using the following reagents. State the positive results.
1) dilute sulfuric acid
2) dilute hydrochloric acid
3) aqueous potassium iodide
4) acidified aqueous KMnO4 / acidified aqueous K2Cr2O7
1)
Cations identified: Ba2+ or Pb2+
Positive result: White precipitate is observed (due to the formation of insoluble BaSO4 or PbSO4).
2)
Cation identified: Pb2+
Positive result: White precipitate is observed (due to the formation of insoluble PbCl2).
3a)
Cation identified: Pb2+
Positive result: Yellow precipitate is observed (due to the formation of insoluble PbI2).
3b)
Cation identified: Cu2+
Positive result: White precipitate in brown solution is observed. (White precipitate is Cu2I2, and brown solution is aqueous I2)
3c)
Cation identified: Oxidising agent (e.g. Fe3+)
Positive result: Brown solution is observed.
4a)
Cation identified: Reducing agents (e.g. Fe2+)
Positive result: Purple solution is decolorised for acidified KMnO4
4b)
Cation identified: Reducing agents (e.g. Fe2+)
Positive result: Orange solution turns green for acidified K2Cr2O7
Describe the test to identify carbonate ions. State the expected observation if carbonate ions are present.
Add any acid to the solution and test the gas evolved using limewater.
Effervescence of a colourless, odourless gas which gave a white precipitate with limewater.
State the type of reaction between limewater and carbon dioxide.
Neutralisation
Describe the test to identify chloride ions. State the expected observation if chloride ions are present.
Add dilute nitric acid followed by aqueous silver nitrate into the solution.
White precipitate of AgCl(s) is formed.
Ionic equation: Ag+ (aq) + Cl- (aq) -> AgCl(s)
Describe the test to identify iodide ions. State the expected observation if iodide ions are present.
Add dilute nitric acid followed by aqueous silver nitrate into the solution.
Yellow precipitate of AgI(s) is formed.
Ionic equation: Ag+ (aq) + I- (aq) -> AgI(s)
Can dilute hydrochloric acid be used to acidify the solution instead of nitric acid when testing for chloride ions or iodide ions? Explain.
No. Chloride ions from the hydrochloric acid would form insoluble silver chloride with silver ions. This defeats the purpose of using silver nitrate to test for presence of chloride or iodide ions.
Suggest a reason why aqueous silver nitrate can be used to test for both chloride and iodide ions.
Chloride and iodide ions form precipitates with silver ions that have different colours.
Describe the confirmation test to identify iodide ions. State the expected observation if iodide ions are present.
Add dilute nitric acid, followed by aqueous lead nitrate into the solution.
Yellow precipitate of PbI2(s) is formed.
Ionic equation: Pb2+ (aq) + 2I- (aq) -> PbI2(s)
