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This practical is to investigate the complex ions of copper (â…¡). It was divided into two parts. In the first part, the empirical formula of hydrated copper (II) sulphate will determined by a dehydration experiment. The second part will study other complex ions in solution
Molar mass is an important data in this practical. Mole mass is the weight of an element in the number of 6.02 * 1023. (1)The molar mass of atoms (M) will be used in this practical included copper (64g/mol), sulfur (32g/mol), oxygen (16g/mol) and hydrogen (1g/mol). There are two formulas will be used in calculation. They are (2) the molar mass of a molecule = the addition of molar mass of each element (M = M1 + M2 + M3), and (3) the mole number = the mass ÷ the molar mass (n = m/M).
copper (II) sulphate crystal concentrated hydrochloride acid
For safety, lab coat and glasses were needed during the practical.
In part A, the first step was to clean the inside of the crucible with a cloth. Then a paper clip was placed in the crucible and the whole dish was weighed on the electronic balance. After putting the crucible on the electric balance, 2-3g of copper sulphate was put in the crucible. The second step was to place the dish on the stand and heat for 5 minutes after the burner was lighted and placed under the stand. The crystals were stirred with the paper clip. Last, the crucible was placed inside the dessicator to cool down. Then the crucible was weighed and heated again as the steps above.
In part B, some copper sulphate and water were put into three conical flasks and shaken to dissolve. Concentrated hydrochloric acid was dropped using a pipette into one flask until the colour changes. In another flask, some ammonia solution was dropped graduate using another pipette and shaken gently. A little solution was dropped first, and then more was added.
The results of the results in part A is shown below in table 1, basic on Lane (2009a).
Heating the copper sulphate
Blue pieces of crystal → white powder
Weighing the mass
Of crucible (+ paper clip)
Of crucible + copper sulphate
After first heating
After second heating
After third heating
Table 1, results in part A (Lane, 2009a)
The results of observation were shown in the table 2.
Copper sulphate +
Concentrated hydrochloric acid
The first few drops
Reaction surface: blue → yellow → (shaken) blue
After more drops
Blue → green.
The first few drops
Occurrence: blue suspension
After more drops
Blue suspension→ indigo → (stewing) little black solid
Table 2, the results of observation in part B
According to the introduction and results, the number of water molecule in copper (II) sulphate crystal could be calculated.
(Formula (1)) M(Cu) = 64g/mol, M(S) = 36g/mol, M(O) = 16g/mol, M(H) = 1g/mol
(Formula (2)) M(CuSO4) = M(Cu) + M(S) + 4*M(O) = 64 + 32 + 4*16 = 160g/mol
M(H2O) = M(O) + 2*M(H) = 16 + 2*1 = 18g/mol
(Reason (9)) m(CuSO4) = m(C) - m(A) = 22.33 - 20.95 = 1.38g
m(H2O) = m(B) - m(C) = 23.11-22.33 = 0.78g
(Formula (3)) n(CuSO4) = m(CuSO4) ÷ M(CuSO4) = 1.38 ÷ 1.38 = 0.008625
n(H2O) = m(H2O) ÷ M(H2O) = 0.78 ÷ 18 = 0.043
(Reason (8)) n(CuSO4) : n(H2O) = 0.08625 : 0.043 = 1 : 4.99 ≈ 1 : 5
(Formula (4)) x ≈ 5
The result of calculation shows that the empirical formula of copper (II) sulphate is CuSO4:5H2O. During the practical, the hating step was down three times in order to make the dehydration more completely. The evaporation of water was almost finished. This practical was successful.
In part B experiment, two reactions were considered. The addition of chlorine acid, in the first few drops, became yellow in the surface and disappeared after shaking because of the slight amount of chloride ions. After adding more solution, the solution turned green. During the reaction, the four chloride ions replace six water molecules and form a tetrachloride copper (II) ion. However, as the equation (7) describes, this reaction is reversible therefore the final production should consists of [Cu(H2O)6]2+ and [CuCl4]2- as well. According to the introduction, the green colour was transmitted because of the exchange of ligands which results in the change in energy gap. This energy gap may be filled up by absorbing the red light. Thus the yellow and blue light transmitted and mixed into green. In this test, the quality of chloride ion on ligand is stronger than the quality of water molecule. In the second test, adding ammonia solution, the first few amount of ammonia can cause the solution into a based environment (more hydroxide ions than hydrogen ions). In another word, the first reaction goes as equation (5) in the introduction and the ammonia acts as hydroxide ions. It is because that little ammonia will ionize in water and produce the hydroxide ions. These hydroxide ions will replace two of the water molecules. When more ammonia was adding, the ions acted more likely as a ligand, as the equation (6). The production made the solution become indigo. Being a ligand, the ammonia ions replaced four of six water molecules and form the copper (II) complex ions, with four ammonia ions and two water molecules. In this test, the ammonia ions show the qualities both on base and ligand which is stronger than water as well. A further practical can be designed to investigate the strength of ligands between chloride ions and ammonia.
In part A, the value of water molecule in each copper sulphate was obtained five which equal to the theoretic value. In the first test of part B, the mixture solution of chloride ions and copper (II) ions is green. During the second test, combing little ammonia solution and copper (II) sulphate solution produces white suspensions. After more addition of ammonia in the second test, the solution becomes indigo.