Complex ions in solution

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This practical is to investigate the complex ions of copper (II). 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). (4)The empirical formula is CuSO4:xH2O(s) ? CuSO4(s) + xH2O (Lane, 2009). The equations in part B are (5)[Cu(H2O)6]2+ + 2NH3 ? [Cu(H2O)4(OH)2] + NH4+, (6)[Cu(H2O)6]2+ + 4NH3 ? [Cu(NH3)4(H20)2]2+ + 4H2O, (7)[Cu(H2O)6]2+ 4Cl- [CuCl4]2- + 6H2O, (Clark, 2003a).

The nature of a complex metal ion is a metal in the centre and surrounded by some molecules or ions with lone pairs (free electron pairs). Those molecules and ions are called ligands. The bones in ligands are dative bones which means the two electrons in the bond is totally contributed by a molecule or ion (Lane, 2009b). According to Clark (2003b), the simplest examples of ligands are water molecule, ammonia and chloride ions.

Copper (Cu, 64g/mol, 1s22s22p63s23p63d104s1) is the twenty-ninth element in the Periodic Table and one of the transition metals, metal from group 2 to 12. (8)Blue copper sulphate is a hydrated salt which has a certain ratio of mole number between the copper (II) sulphate and water molecules in the solid crystal. That is why copper sulphate can be expressed by the formula CuSO4:xH2O, where x will be determined in the first part in this practical. (9)When heated, it will lose the water molecule and become white powder which is called anhydrous salt. By the differences of mass, before and after heating, the mass of water and then the value of x can be calculated. Theoretically, the formula of this hydrated salt should be CuSO4:5H2O. When copper (II) sulphate dissolves in water, the ionization will happen and the copper (II) ions in solution will form.

When Cu2+ forms, 4s electron and one of the 3d electrons have been lost. After that, the empty 4s and 4p electron shells will have been used for accepting lone pairs. As Clark (2003a) stated, [Cu(H2O)6]2+, a copper (II) complex ion with six water molecules in solution, is the typical blue hex aqua copper (II) ion. If amount of molecules or ions with lone pairs dissolves in the solution, theses ligands may replace the water molecule ligands. In the other words, the reaction will take place and the production can be noticed. As an example, each of four chloride ions with lone pair will form the dative bonds with copper as figure 1, (Clark 2003b). However, the chloride ions are too big to fit in six spaces therefore the copper (II) complex ions with four chloride ions, tetrachloride copper (II) ions, will be produced (Clark, 2003b).

Lister (2000) insists that the colour of complex metal ions in solution is caused when the electrons of ligands in d electron orbital (electron shell) move to a higher energy level orbital and form a gap if energy. He claimed that the electromagnetic energy will be taken in by the electrons to fill in the energy gap (Lister, 2000). The equation E = h*v is given by Lister (2000) where E is the energy gap, h is the Planck's constant and v is the frequency of energy. If v is within the frequency range of visible light, the complex will be in the transmitted colour(Lister, 2000). An example given by Clark (2003c) is that the colour of copper (II) sulphate solution is blue. He explained that the electrons in 4d orbital of hexaqua copper (II) ions, with six water molecules, spilt into two groups, one in higher energy (Figure 2, Clark, 2003c). The absorption of hexaqua copper (II) is shown in Figure 3 by Clark (2003c). A further statement of Clark indicated that E is determined by both the nature of ligands and the nature of transition metals. Therefore, the colour may change when the ligand exchange reactions take place.

Two of these reactions will be studied in this practical. One is copper sulphate and ammonia. Another is copper sulphate dealing with hydrochloric acid.

Methods and equipment

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.


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.


  • Clark, J (2003a) [online] Copper Available at: [Accessed date 25/12/09]
  • Clark, J (2003b) [online] An Introduction to Complex Metal Ions Available at: [Accessed date 25/12/09]
  • Clark, J (2003b) [online] The Colour of Complex Metal Ions Available at: [Accessed date 25/12/09]
  • Lane, R (2009a) Chemistry Practical II. IFY class handout
  • Lane, R (2009b) Kinds of Bond. IFY class note
  • Lister, T. and Renshaw, J. (2000) Chemistry For Advanced Level (3rd Edition) Ltd: Stanley Thrones