Acid and base- buffer question
I understand the whole question asides from one thing about it.
>When i calculate the concentration of the salt, and since i need to get the mass of it, i realise i gotta convert it into moles so i can then get it into mass.
>So this means that ill have to multiply the concentration of the salt with the volume of the salt.
>But you see when it said the the sodium ethanoate was added to the weak acid to create the buffer solution it said that the volume remained constant at 400cm3.
>This confuses me, surely the volume woul increase since a substance was added, and i need to know that olume in order for me to carry on in the answer to the question.
>But apparently the volume of it now becomes 400cm3 too, i checked the markscheme, and i dont really understand why.
>Wasnt the weak acid 400cm3? Isnt saying that the salt also being of the same value suggesting that the whole 400cm3 is now the salt and not the weak acid?
>I dont understand this point in the answer, so im hoping someone can explain it!
Many thanks
>When i calculate the concentration of the salt, and since i need to get the mass of it, i realise i gotta convert it into moles so i can then get it into mass.
>So this means that ill have to multiply the concentration of the salt with the volume of the salt.
>But you see when it said the the sodium ethanoate was added to the weak acid to create the buffer solution it said that the volume remained constant at 400cm3.
>This confuses me, surely the volume woul increase since a substance was added, and i need to know that olume in order for me to carry on in the answer to the question.
>But apparently the volume of it now becomes 400cm3 too, i checked the markscheme, and i dont really understand why.
>Wasnt the weak acid 400cm3? Isnt saying that the salt also being of the same value suggesting that the whole 400cm3 is now the salt and not the weak acid?
>I dont understand this point in the answer, so im hoping someone can explain it!
Many thanks
Bear in mind that sodium ethanoate is a solid salt, and could be added without pre-dissolving it. This would mean there is unlikely to be significant change in volume. The question is asking you to consider that change in volume to be so negligible that you should consider it to be unchanged.
Original post by TutorsChemistry
Bear in mind that sodium ethanoate is a solid salt, and could be added without pre-dissolving it. This would mean there is unlikely to be significant change in volume. The question is asking you to consider that change in volume to be so negligible that you should consider it to be unchanged.
Sorry for the tardy response, but wouldnt that essentially mean that the volume thereofre of the slat should be 0 cm3 instead of assigning it the whole volume of 400cm3 of the weak acid?
No, they were just *assuming* after you added the sodium ethanoate the total volume remain the same. In effect the volume doesn't change but you adding more base into the solution.
In an ideal world, you can't achieve this without increasing the overall volume.
However, in term of the calculation. you are doing (0.35 * 400/1000) = Moles of addition sodium ethanoate that made the solution with ph 5.0.
Conclusion
This is just a question with an assumption that sodium ethanoate dissolve fully without changing the total volume.
In an ideal world, you can't achieve this without increasing the overall volume.
However, in term of the calculation. you are doing (0.35 * 400/1000) = Moles of addition sodium ethanoate that made the solution with ph 5.0.
Conclusion
This is just a question with an assumption that sodium ethanoate dissolve fully without changing the total volume.
Original post by Yme2day
Sorry for the tardy response, but wouldnt that essentially mean that the volume thereofre of the slat should be 0 cm3 instead of assigning it the whole volume of 400cm3 of the weak acid?
When making calculations for the buffer using the acid dissociation constant we are considering the concentration of salt anion (and everything else) in the total volume, not in the volume we added.
Original post by Yme2day
I understand the whole question asides from one thing about it.
>When i calculate the concentration of the salt, and since i need to get the mass of it, i realise i gotta convert it into moles so i can then get it into mass.
>So this means that ill have to multiply the concentration of the salt with the volume of the salt.
>But you see when it said the the sodium ethanoate was added to the weak acid to create the buffer solution it said that the volume remained constant at 400cm3.
>This confuses me, surely the volume woul increase since a substance was added, and i need to know that olume in order for me to carry on in the answer to the question.
>But apparently the volume of it now becomes 400cm3 too, i checked the markscheme, and i dont really understand why.
>Wasnt the weak acid 400cm3? Isnt saying that the salt also being of the same value suggesting that the whole 400cm3 is now the salt and not the weak acid?
>I dont understand this point in the answer, so im hoping someone can explain it!
Many thanks
>When i calculate the concentration of the salt, and since i need to get the mass of it, i realise i gotta convert it into moles so i can then get it into mass.
>So this means that ill have to multiply the concentration of the salt with the volume of the salt.
>But you see when it said the the sodium ethanoate was added to the weak acid to create the buffer solution it said that the volume remained constant at 400cm3.
>This confuses me, surely the volume woul increase since a substance was added, and i need to know that olume in order for me to carry on in the answer to the question.
>But apparently the volume of it now becomes 400cm3 too, i checked the markscheme, and i dont really understand why.
>Wasnt the weak acid 400cm3? Isnt saying that the salt also being of the same value suggesting that the whole 400cm3 is now the salt and not the weak acid?
>I dont understand this point in the answer, so im hoping someone can explain it!
Many thanks
The salt is a solid which is going to essentially be dissolved in the 400 cm3 solution. The question wants you to assume that upon adding the salt to the ethanoic acid the volume won't change which is a reasonable assumption as you are not adding another aqueous solution but instead adding a solid. So now the volume the salt ions will occupy is 400cm^3 which is why you use this value.
We are not talking about the overall volume of the acting salt particles (like adding up the volume of their spheres) but we are talking about the volume of the solution instead.
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