Fertiliser Practical

[frog1944]

Hi,

I'm a bit confused about the fertiliser practical and why we do each step, this is what I currently have;

1. Grind a small amount of fertiliser using a mortar and pestle.
2. Accurately weigh 1.0 g of fertiliser in a beaker using electronic balance.
3. Add 10 mL 2M HCl and 50 mL water and stir to dissolve all fertiliser. (Acid removes stuff that could precipitate with Barium).
4. Filter the mixture and rinse thoroughly.
5. Heat mixture gently on a hot-plate and slowly add barium chloride solution until no more precipitate forms.
6. Add acetone (to take act as a dehydrating agent, and coagulate molecules)
6. Filter through a weighed quantitative filter paper. Wash the precipitate with warm water (why warm water?) and dry the precipitate thoroughly.
7. Weigh the precipitate and filter paper, calculate the mass of  collected.

Are the reasons for these steps correct? Why do we wash with warm water in step 6?

Thanks

[Natasha.97]

Hi!

3. Add 10 mL 2M HCl and 50 mL water and stir to dissolve all fertiliser. (Acid removes stuff that could precipitate with Barium).

Yep! It neutralises CO₃^2-, which would otherwise form BaCO₃ and compromise solubility of the fertiliser.
2H⁺_(aq) + CO₃^2-_(aq) → CO_2(g) + H₂O_(l)

5. Heat mixture gently on a hot-plate and slowly add barium chloride solution until no more precipitate forms.

For this, you could add that heat improves solubility, and that the particles would be coarse if heated slowly (if the particles are too fine, the precipitate may be lost in the filtration step, compromising validity and accuracy).

6. Add acetone (to take act as a dehydrating agent, and coagulate molecules)

Correct!

Why do we wash with warm water in step 6?

What I've got in my notes is that it ensures all the precipitate has been transferred from the beaker to the filter paper.

Hope this helps

[frog1944]

Thanks, that helps a lot! For the heating of the mixture in step 5, by making the particles coarse does that promote flocculation? Also, for the acetone which can coagulate the molecules, is that the same as flocculation?

[MisterNeo]

Thanks, that helps a lot! For the heating of the mixture in step 5, by making the particles coarse does that promote flocculation? Also, for the acetone which can coagulate the molecules, is that the same as flocculation?

Coagulation isn't really the same as flocculation as they occur separately. The precipitate formed usually clumps after most of the solution is drained through the filter paper. For these types of questions, I wouldn't go into this much depth but just refer to the collection of barium sulfate precipitate using filter paper.

[frog1944]

Ok, thanks