The Student Room Group

chemistry precipitation questions

1) a student observes a reaction in their lab between two aqueous solutions but the solubility table indicates that both products are aqueous and thus a reaction should not have been observed. propose a source of error which may explain their results

(my guess would be impurities in the samples?)

2) A student observes 'no visible reaction' for their results, yet the solubility table indicates that one of the products is insoluble. Propose a source of error that may explain their results. (The student did mix the correct chemicals)

(my guess would be one of the preciptates reacted with like an acid in excess?)
Original post by a_level185
1) a student observes a reaction in their lab between two aqueous solutions but the solubility table indicates that both products are aqueous and thus a reaction should not have been observed. propose a source of error which may explain their results

(my guess would be impurities in the samples?)

2) A student observes 'no visible reaction' for their results, yet the solubility table indicates that one of the products is insoluble. Propose a source of error that may explain their results. (The student did mix the correct chemicals)

(my guess would be one of the preciptates reacted with like an acid in excess?)

Did you know that solubility values are variable? What affects the value?

Also, what does "solubility" mean? What is "saturated"? Can a solution not be saturated?
Original post by Pigster
Did you know that solubility values are variable? What affects the value?

Also, what does "solubility" mean? What is "saturated"? Can a solution not be saturated?

yeahhh i know there is a table of solubility values

solubility is when something dissociates and is hydrated, so ions forming attraction with the polar charges on water.

and then a saturated solution is when its dissolved the maximum amount of solute so the rest left over is the solid solvent?

i guess im confused because it says source of error and i dont see how that ties in as an error unless the error is adding too much of one solute?

(also this isn't homework its just fascination i was looking at some chemistry work set in america and came across this so if you were to give me the answer then it doesn't count towards anything but i do appreciate you helping me get there! i feel silly for not knowing since i got an A in chemistry)
Original post by a_level185
yeahhh i know there is a table of solubility values

solubility is when something dissociates and is hydrated, so ions forming attraction with the polar charges on water.

and then a saturated solution is when its dissolved the maximum amount of solute so the rest left over is the solid solvent?

i guess im confused because it says source of error and i dont see how that ties in as an error unless the error is adding too much of one solute?

(also this isn't homework its just fascination i was looking at some chemistry work set in america and came across this so if you were to give me the answer then it doesn't count towards anything but i do appreciate you helping me get there! i feel silly for not knowing since i got an A in chemistry)

You still didn't answer the 1st Q: what affects solubility?

As for my second point, re saturation: if a substance, MX, has a solubility of 1 mol dm^-3 and you react 500 cm^3 of 0.5 mol dm^-3 HX with 500 cm^3 of 0.5 mol dm^-3 MOH you'll make 0.5 mol of MX and the total volume is 1 dm^3 - it isn't saturated and therefore won't throw a ppt. Whereas if the solubility of MX was 0.2 mol dm^-3, it would be past saturation and it would throw a ppt.
Original post by a_level185
1) a student observes a reaction in their lab between two aqueous solutions but the solubility table indicates that both products are aqueous and thus a reaction should not have been observed. propose a source of error which may explain their results

(my guess would be impurities in the samples?)

2) A student observes 'no visible reaction' for their results, yet the solubility table indicates that one of the products is insoluble. Propose a source of error that may explain their results. (The student did mix the correct chemicals)

(my guess would be one of the preciptates reacted with like an acid in excess?)

Original post by Pigster
Did you know that solubility values are variable? What affects the value?

Also, what does "solubility" mean? What is "saturated"? Can a solution not be saturated?

Based on @pigster response.. It could be due to reaction between the aqueous solutions being endothermic as a result temperature of solution had dropped, and because generally solubility decreases with decrease in temperature, crystals were formed.. which could be the observation for the first one..
An unexpected precipitate can sometimes occur if solutions are prepared too early in advance. As they stand, they absorb carbon dioxide from the air which leads to the formation of the carbonate ion which forms precipitates with a number of metal ions.

Quick Reply

Latest