Practical 2 - estimation of copper in copper (II) salts Flashcards
Aim
To determine the % by mass of copper in copper (II) sulfate crystals
Apparatus
3 decimal place mass balance
Weighing boat
2 x 50cm^3 beakers
250cm^3 beaker
3 x 250cm^3 conical flask
250cm^3 volumetric flask with stopper
25cm^3 bulb pipette
Pipette filler
Burette, funnel and stand
White tile
Wash bottle of de-ionised water
2x Pasteur pipettes
Chemicals
CuSO4.H2O crystals
0.1moldm^-3 Na2S2O3 solution
0.2% starch solution
KI powder
Which substances can be hazardous in this practical and why?
CuSO4.H2O —> harmful, dangerous to the environment
KI powered —> irritant
Method
1.) accurately weigh out about 6g copper (II) sulfate crystals into a weighing boat
2.) use the copper (II) sulfate crystals and deionised water to make up 250cm^3 of standardised copper (II) sulfate solution
3.) pipette 25cm^3 of this solution into a conical flask
4.) add 1.5g potassium iodide (an excess to ensure that all of the copper ions react) and swirl thoroughly
5.) titrate this solution with standard o.1moldm^3 Na2S2O3 (sodium thiosulfate) in a burette. When the iodine colour fades and turns to a straw-like yellow (just an estimated point), add 1cm^3 starch indicator (turns blue-black)
6.) use your titration data to calculate the % by mass of copper in the copper (II) sulfate crystals
What can be seen at the end of the titration?
The solution turns flesh coloured
What does it mean at the end of the titration when the solution has turned flesh coloured?
All of the iodine has reacted
Sodium thiosulfate
Na2S2O3
Na2S2O3
Sodium thiosulfate
Why is the amount of potassium iodide added just an estimate?
It’s in excess to ensure that all of the copper ions react
Equations for the reactions taking place during this experiment
2Cu2+ + 4I- —> 2CuI + I2
I2 + 2S2O3^2- —> 2I- + S4O6^2-
What does the percentage of copper in copper (II) sulfate crystals depend on?
The purity of the sample
