Aim: To know the procedure of measuring the accurate mass of a solid To calculate the number of moles of an unknown mass from its mass and molecular mass To know how to dilute a solution and the effect of dilution on the solution’s absorbance Materials: Copper Sulfate Coloured solution – Potassium Manganate (KMnO4) Hotplate Method: Part A: The Formula of Hydrated Copper (II) Sulfate Firstly, about 1. 0 g of hydrated copper (II) sulfate was put in the weighing bottle and the colour was noted.
The mass of capped weighing bottle and hydrated copper (II) sulfate in it were measured and recorded. Then, the copper (II) sulfate was transferred into a casserole and the mass of the empty weighing bottle was measured and recorded. This method is necessary as to ensure the mass of copper (II) sulfate is mostly accurate and as to reduce the chance of spilling the chemicals while weighing it. Next, the copper (II) sulfate in the casserole was heated using a hot plate until the colour turned as a pale blue.
After that, the casserole was removed from the hot plate and it was allowed to cool down. After a while, the content of the casserole was transferred back into the weighing bottle and its mass was measured and recorded again as to get the mass of dehydrated copper (II) sulfate which was in white. Part B: Dilution of a Coloured Solution A beaker was used to collect roughly about 20cm3 of the coloured solution. About 10 cm3 of the coloured solution was transferred into a volumetric flask using a pipette and deionized water was added until it reached the mark.
The mixture was then shaken well. Before the coloured solution was measured for its absorbance, deionized water was first placed in the ultraviolet-visible (UVV) spectrophotomter as to calibrate the UVV spectrophotometer to zero. Then only some of the diluted solution in the volumetric flask was transferred into a cuvette cell using a dropper and its absorbance was measured and recorded. After that, the balance of the coloured solution in the beaker was also transferred into a cuvette cell and its absorbance was measured and recorded.
The balance of the coloured solution in the beaker represents a concentrated solution and it was used to compare the absorbance between a concentrated solution and a diluted solution. Results: Part A Table 1: Data measurement Mass of capped weighing bottle + hydrated (blue) copper (II) sulfate30. 887 g Mass of empty (capped) weighing bottle27. 879 g Mass of hydrated copper (II) sulfate1. 008 g Mass of capped weighing bottle + dehydrated (white) copper (II) sulfate30. 512 g Mass of water0. 375 g Moles of white copper (II) sulfate0. 003956 mol
Moles of water0. 0208 mol Moles of water/ moles of copper (II) sulfate5. 2578 mol Part B Table 2: Coloured Solutions and Its Absorbance Absorbance of concentrated coloured solution1. 950 Absorbance of diluted coloured solution: 1st reading 2nd reading 3rd reading Average reading of diluted coloured solution 0. 220 0. 221 0. 221 0. 221 (3 d. p) Discussion: Part A: CuSO4. xH2O Q1. What is the formula of hydrated copper (II) sulfate? Moles of copper (II) sulfate = 0. 003956 mol Moles of water = 0. 0208 mol Ratio of moles of copper (II) sulfate and water =
CuSO4 : H2O 0. 003956/0. 003956 : 0. 0208/0. 003956 1:5. 3 Approximately, 1 molecules of copper (II) sulfate will have 5 molecules of water. Value of x = 5 Formula of hydrated copper (II) sulfate = CuSO4. 5H2O Part B: The result shows that the diluted coloured solution has a lower absorbance value compared to the concentrated coloured solution. This is because absorbance of a solution is directly related to its concentration. References: Silberberg M. S 2006, Chemistry- The Molecular Nature of Matter and Change, 4th edition, McGraw-Hill, New York.