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Titration question percentage purity?

I’ve almost done it but need to find the percentage purity. How do you do that?
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It’s the 4th question
Reply 2
Avatar for Ðeggs
Ðeggs
22
You can calculate the moles of NaHCO3 from the mass and the Mr given in the question, as well as the number of moles of NaHCO3 that react from the number of moles of hydrochloric acid. Then consider the total number of moles vs the actual number of moles that react and write the ratio as a percentage. Remember to times the ratio of moles from HCl by ten to have both moles in 250cm^3 of solution.
(edited 4 years ago)
Original post by Deggs_14
You can calculate the moles of NaHCO3 from the mass and the Mr given in the question, as well as the number of moles of NaHCO3 that react from the number of moles of hydrochloric acid. Then consider the total number of moles vs the actual number of moles that react and write the ratio as a percentage. Remember to times the ratio of moles from HCl by ten to have both moles in 250cm^3 of solution.

I'm assuimng that it's the fully-reacted NaHCO3 which is pure then, right? Why is it that calculating the % of NaHCO3 that reacted gives you the % of pure NaHCO3? Is it because only the pure NaHCO3 would react?

Thanks in advance
Reply 4
Avatar for Ðeggs
Ðeggs
22
Original post by MedicPls
I'm assuimng that it's the fully-reacted NaHCO3 which is pure then, right? Why is it that calculating the % of NaHCO3 that reacted gives you the % of pure NaHCO3? Is it because only the pure NaHCO3 would react?

Thanks in advance


Well the question states the NaHCO3 was impure, so a part of that mass will be made of impurities. But only the NaHCO3 itself will react with the HCl. So the ratio of the two moles determines the percentage of the mass that actually reacted, and hence the purity.
(edited 4 years ago)
Original post by Deggs_14
Well the question states the NaHCO3 was impure, so a part of that mass will be made of impurities. But only the NaHCO3 itself will react with the HCl. So the ratio of the two moles determines the percentage of the mass that actually reacted, and hence the purity.

right so part of the moles of NaHCO3 was impure
so the total moles of it - moles that reacted = moles of impurity remaining?
Reply 6
Avatar for Ðeggs
Ðeggs
22
Original post by MedicPls
right so part of the moles of NaHCO3 was impure
so the total moles of it - moles that reacted = moles of impurity remaining?


(Moles that reacted / total moles of compound ) x 100.
Original post by Deggs_14
(Moles that reacted / total moles of compound ) x 100.

yeah that gives you the % purity right?
Yesssi got it!!
Original post by ArtmisKco
Yesssi got it!!

hi, could you tell me what the correct % purity is please so i can check my answer :smile:
Of course! It will be 2.41/3.15 *100
So the answer is 76.5%
Original post by ArtmisKco
Of course! It will be 2.41/3.15 *100
So the answer is 76.5%

is that the correct answer for definite? I got 43.7 % :frown:

With my working out i see how i could have gotten 76.5% but I would need to use ( mass reacted / total mass ) instead of ( moles reacted / total moles ). How do you know which one should be used to calculate % purity? I would have thought you use the moles equation.

Thank you in advance :smile:

edit: i understand now! Can't use moles equation because calculating moles of the original impure substance is impossible, since you don't know the actual Mr of the impure substance.
(edited 4 years ago)

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