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Identifying Unknowns Practical

Hi, next Wednesday I have my practical endorsement for A Level OCR chemistry (PAG 4.3) and for homework I am meant to be planning a method for the practical. Can anyone help me out?
In the practical we have to identify 5 unknowns, A-E. They are in no particular order:
nitric acid, HNO3(aq)
potassium iodide, KI(aq)
sodium carbonate, Na2CO3(aq)
sodium chloride, NaCl(aq)
silver nitrate, AgNO3(aq)

So far, I know that I should first carry out a carbonate test, adding HNO3 to each, and whichever shows effervescence is the carbonate.
Then, I will do a halide test, adding AgNO3 to the remaining unknowns, whichever forms a white precipitate will be the NaCl and whichever forms the yellow precipitate will be the KI.

I am unsure how to identify AgNO3 and HNO3. I know that both of these substances are used in the tests mentioned above- does that have any significance? Or are there any other separate tests I can do to identify them?

Thanks :smile:
I think you've basically got it, but I assume you won't know which the HNO3 or AgNO3 are initially, meaning you'll have to react some compounds first to find out which they are. For example the two unknowns that form a white preciptate have to be AgNO3 and NaCl as mentioned, but you won't know which is which until you react each of these with another unknown to form the yellow precipitate (KI as mentioned) and the NaCl will form a solution with KI, but no precipitate. Now you know those three, you know the remaining two are Na2CO3 and HNO3, so you can react each of the two unknowns with AgNO3 - the Na2CO3 will form a yellow precipitate of AgCO3 (rememeber most carbonates are insoluble, so the colour doesn't actually matter here) - there may be another way the spec wants you to do this however.

You are correct with what you said, but I think there is a fundamental logic error, because you can't just add HNO3 or AgNO3 if they are all unknown initially.

And I haven't come across any tests for HNO3 or AgNO3 explicitly, but there may be some.
Reply 2
Original post by richmaths
I think you've basically got it, but I assume you won't know which the HNO3 or AgNO3 are initially, meaning you'll have to react some compounds first to find out which they are. For example the two unknowns that form a white preciptate have to be AgNO3 and NaCl as mentioned, but you won't know which is which until you react each of these with another unknown to form the yellow precipitate (KI as mentioned) and the NaCl will form a solution with KI, but no precipitate. Now you know those three, you know the remaining two are Na2CO3 and HNO3, so you can react each of the two unknowns with AgNO3 - the Na2CO3 will form a yellow precipitate of AgCO3 (rememeber most carbonates are insoluble, so the colour doesn't actually matter here) - there may be another way the spec wants you to do this however.

You are correct with what you said, but I think there is a fundamental logic error, because you can't just add HNO3 or AgNO3 if they are all unknown initially.

And I haven't come across any tests for HNO3 or AgNO3 explicitly, but there may be some.

Hi, thanks so much for your response! I have been looking at the practical sheet and it says that we will also be given pH indicator paper. Both HNO3 and AgNO3 are acidic, however HNO3 has a much lower pH of 1 than AgNO3 which has a pH of 4.3. Do you think that this difference in pH will be clear on indicator paper? That would be a nice quick way of identifying them but I don't know how clear the difference in colour will be?
Original post by ConnieFox
Hi, thanks so much for your response! I have been looking at the practical sheet and it says that we will also be given pH indicator paper. Both HNO3 and AgNO3 are acidic, however HNO3 has a much lower pH of 1 than AgNO3 which has a pH of 4.3. Do you think that this difference in pH will be clear on indicator paper? That would be a nice quick way of identifying them but I don't know how clear the difference in colour will be?

Oh yeah that is good! Yeah that should be a clear difference, HNO3 should produce dark red paper, and AgNO3 should be more orange/ yellow or maybe slightly green :smile:
Reply 4
Original post by richmaths
Oh yeah that is good! Yeah that should be a clear difference, HNO3 should produce dark red paper, and AgNO3 should be more orange/ yellow or maybe slightly green :smile:

Great, thanks so much for your help! :smile:
Original post by ConnieFox
Great, thanks so much for your help! :smile:

Happy to help :smile: It is fun discussing chemistry problems!
Reply 6
Original post by ConnieFox
Hi, next Wednesday I have my practical endorsement for A Level OCR chemistry (PAG 4.3) and for homework I am meant to be planning a method for the practical. Can anyone help me out?
In the practical we have to identify 5 unknowns, A-E. They are in no particular order:
nitric acid, HNO3(aq)
potassium iodide, KI(aq)
sodium carbonate, Na2CO3(aq)
sodium chloride, NaCl(aq)
silver nitrate, AgNO3(aq)
So far, I know that I should first carry out a carbonate test, adding HNO3 to each, and whichever shows effervescence is the carbonate.
Then, I will do a halide test, adding AgNO3 to the remaining unknowns, whichever forms a white precipitate will be the NaCl and whichever forms the yellow precipitate will be the KI.
I am unsure how to identify AgNO3 and HNO3. I know that both of these substances are used in the tests mentioned above- does that have any significance? Or are there any other separate tests I can do to identify them?
Thanks :smile:

hi there i know its a long shot but do you have the method you used?

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