Sulfate content of fertilizer prac?? (1 Viewer)

[leesh95]

I had to do this prac in class and our results were very very off. We had a diference of about 5% from the original results.

I don't know what went wrong??
I know we spilled about 2 drops but it wasn't enough to affect the results this substantially.

I know that I stirred the mixture quite a lot which according to our teacher would have agitated the particles and resulted in them being very small. However when we filtered our solution using a sintered glass crucible it didn't seem cloudy.

Also when we cooled our precipitate some of it stuck to the bottom of the beaker and we used about a whole bottle of de-ionised water to make it unstuck and then half a bottle of de-ionised water to wash the beaker and make sure none of the precipitate was left behind. Our teacher said this could have also affected our result but he didn't elaborate.

Can somebody please suggest anything else that could have gone wrong??

 

[madharris]

Possibly:
Not all of the precipitate was dried (water was still there when precipitate was weighted)
Contamination of precipitate with substances absorbed from solution during precipitation
Loss of precipitate passing through filter paper as particles formed are small
Incomplete transfer of precipitate to filter funnel (spillage, precipitate sticking to walls of beaker)

that's all i can think of at the moment

 

[leesh95]

Can contamination cause the precipitate to weigh less than it should? Because ours was a lot less than the original amount.

Also is it possible that the some of the precipitate passed through the sintered glass crucible but the filtrate collected didn't look cloudy.

 

[Rawf]

Can contamination cause the precipitate to weigh less than it should? Because ours was a lot less than the original amount.

Also is it possible that the some of the precipitate passed through the sintered glass crucible but the filtrate collected didn't look cloudy.

This is why you're supposed to repeat the experiment. Tbh, I didn't really read the whole thing you posted, however, it's not very often that you'll achieve the same results as those given in textbooks/whatever. I think every practical we did in school was dodgy. Plus, it could be due to the differences in fertilisers... the sulfate contents may vary, just like any other element/ions. Do you expect every bottle of orange juice to have the same amount of citric acid?

 

[leesh95]

Everyone got the same fertiliser from the same bag and on the back it said the sulfate content was about 26.8% and many people got results quite close to that value

 

[Kimyia]

Haha you don't want to know how far off my classes values were for that prac, but madharris has covered most of the errors that I can think of.

 

[someth1ng]

Not all of the precipitate was dried (water was still there when precipitate was weighted)
Contamination of precipitate with substances absorbed from solution during precipitation
Loss of precipitate passing through filter paper as particles formed are small
Incomplete transfer of precipitate to filter funnel (spillage, precipitate sticking to walls of beaker)

On top of madharris' list:
Particles can adhere to the walls of the sintered glass funnel, the collection flask and beaker.
Not only sulfate in fertiliser was precipitated, there can be other substances that could form precipitates with barium ions (eg sulfite, phosphates etc)
The sulfate was not fully dissolved or was already insoluble (such as BaSO4) or were trapped in insoluble parts of fertiliser, avoiding dissolution.
BaSO4 is very slightly soluble, some BaSO4 was dissolved and hence, would pass filtration entirely.
When the fertiliser was crushed, it must be re-weighed as there would be some loss of fertiliser.
It was assumed that the single balance used was correct and the value was not confirmed against other balances making the readings unreliable and hence, high chance of undetected error.
Filter paper alone can contain some very small amounts of water, and after filtration, when heated, not all water may be removed or it may be over heated - possibly losing more water than previously weighed.
As the BaSO4 is dried, it becomes brittle and cracks making it susceptible to small losses in BaSO4 solid.
BaSO4 can easily adhere to hands when removed and transferred such as from the sintered glass funnel to the heating oven causing loss of BaSO4.