Gravimetic Analysis

[confusedperson123]

Hey SAC coming up soon I'm sure this SAC would be similar to most schools too

Moving on,

1) I got 0.995g of fertiliser which has a label of xx%
2) Add 50mL deionised water and stir to dissolve the sample. This is then poured in a 400mL beaker using a filter paper.
3) Add 2.5mL of 2M HCl to the filtered solution and add more water so the total volume is 200mL
4) Add 15mL barium chloride solution (excess) to the hot solution. A white precipitate of barium sulfate will form
5) weigh a glass crucible = 30.478
--vacuum filtration is used--
6) final weight = 31.668; thus, 1.19g of precipitate is formed.

Questions:
1. So what does the HCl actually do?
2. We presume that the dissolved particle is pure ammonia sulfate? would this be a systematic error?
3. Should my answer be in 3 SF since my mass of fertiliser is in 0.995g?
4. What else should i look out for? What are classic examples of systematic/random error that is evident in this experiment?

Thanks

[longy1991]

1. HCl removes the other ions in the fertiliser that would precipitate out upon addition of Ba2+ ions.
2. No idea what you are talking about
3. Yep, unless you've used another value somewhere that has 2 or less sig figs.
4. assume no sulfates were filtered out originally, nothing is ever 100% insoluble, did you weigh to constant mass?, splashing, filtering is always messy and solution is often lost, etc.

[confusedperson123]

thanks a lot. also in this sac n(barium sulfate) = n(sulfate)  IF the precipitate is potassium sulfate will the mole of sulfate be divided by two? K2SO4

[superflya]

1 mole of present in 1 mole so no need to divide by 2.

[physics]

1 mole of present in 1 mole so no need to divide by 2.

i suppose that means its a spectator ion....its good to mention that in your sac...(i lost 1 mark due to not mentioning it)