(Q27b from the 2003 exam)
I'm stuck on the second part. I know its about not repeating it so its not reliable, but how do I write that for 3 marks?
Hey there, I am by no means expert at HSC chem but I'll try my best... To get 3 marks, I believe you need to explain the relationship between repetition and reliability and make judgements based on your explanation of the matter. You might want to talk about how reliability depends on repetition of the procedure.
So I'll just start you off with a few dot-points:
- fertiliser may not have been completely soluble in water, therefore mixtures must be filtered
- Acid should have been used to help dissolve and remove carbonate ions and prevent phosphate ions from precipitating
- Use Ba_2Cl to precipitate sulfates (explain why)
- Discuss the unreliability of the experiments:- technique used to collect the precipitate that can cause precipitates of formed by other impurities, lack of proof that 50mL of Barium Chloride was sufficient to precipitate all sulfate ions..
- Then discuss how to make the experiment more valid and increase the reliability by averaging out results from repeated experiment (also note that if the procedure is not done properly, the results are not valid)..
hopefully that helps