4.1 Flashcards
.1 Identifying unknowns (1)• To identify the positive and negative inorganic ions in a mixture of salts, using appropriate apparatus and procedures.
You are given a mixture of two salts of the same Group 2 metal. You will first use qualitative tests to identify the negative ions in the two salts. These tests make use of reactions and observations that are specific for particular ions.
Next, you will identify the Group 2 metal ion in the two salts, making use of the fact that salts of different Group 2 metals have different solubilities. You will mix a solution of the unknown salts with two known salts, and observe whether any precipitation occurs. Using solubility data provided, you will be able to identify the positive ion.
A mixture of barium chloride, BaCl2 and calcium carbonate, CaCO3.
Toxic if swallowed
Causes serious eye irritation
Dilute nitric acid, HNO3(aq)
Causes skin irritation and serious eye irritation
Silver nitrate solution, AgNO3(aq)
No hazard classification
Can stain the skin
Dilute ammonia, NH3(aq)
Causes skin and eye irritation
Aqueous sulfuric acid, H2SO4(aq)
Causes skin irritation
Causes serious eye irritation
Potassium chromate solution, K2CrO4(aq)
May cause cancer
May cause genetic defects
May cause an allergic skin reaction
Equipment
- wash bottle containing distilled (or de-ionised water)
- measuring cylinder (25 cm3)
- beaker (100 cm3)
- glass rod
- filter paper
- conical flask (100 cm3)• filter funnel
- test-tube holder
- four test tubes
- dropping pipettes
- spatula
Health & Safety
- Wear eye protection (goggles) throughout.
* Wear protective gloves when handling K2CrO4.
Procedure part 1
: Identifying the negative ions (anions) present in Mixture A
- Using a measuring cylinder, transfer about 25 cm3 of distilled (or de-ionised) water into a beaker.
- Tip Mixture A into the water in the beaker. Stir the mixture with a glass rod for about 1 minute.
- Filter the mixture into a conical flask.
- Add about 1 cm depth of the filtrate to a test tube.
- Add five drops of HNO3(aq) followed by ten drops of AgNO3(aq). Record your observations.
- Add NH3(aq) dropwise into the test tube. Keep adding NH3(aq) until no further change occurs. Record your observations.
- Add a spatula measure of the solid residue from Step 3 to a test tube.
- Add about 1 cm depth of HNO3(aq) then keep adding HNO3(aq) until no further change occurs. Record your observations.
- Suggest the formulae for the two negative ions present in mixture A using your observations. If you do not know what your observations mean, try to find the relevant information in your textbook or online.
- Write the equations for the reactions you have observed.
Procedure part 2:
Identifying the Group 2 positive ion (cation) present in Mixture A
- Add about 1 cm depth of the filtrate from Step 3 to a new test tube.
- Add about the same volume of H2SO4(aq). Record your observations.
- Add about 1 cm depth of the filtrate from Step 3 to a new test tube.
- Add about the same volume of K2CrO4(aq). Record your observations.
- Suggest the formula of the Group 2 positive ion present in mixture A using your observations and the solubility (in water) information from the table below.
Addition of nitric acid to the residue:
BaCO3(aq) + 2HNO3 (aq) Ba(NO3)2(aq) + CO2(g) + H2O(l)
Addition of H2SO4 to filtrate:
Ba2+(aq) + SO42(aq) BaSO4(s)
Addition of K2CrO4 to filtrate:
Ba2+(aq) + CrO4(aq) BaCrO4(s)
action Add 5 drops HNO3 to filtrate, followed by 10 drops of AgNO3.
White precipitate forms.
