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Buffer solutions- A2

Hey! would be a reeaally big help if someone could explain to me this chemistry question on buffer solutions!

A chemist prepares a buffer solution by mixing together the following.

200cm^3 of 3.20moldm^-3 HCOOH (Ka=1.70X10^-4 moldm^-3) and 800cm^3 of 0.5 moldm^-3 NaOH.

The volume of the buffer solution is 1.00 dm^3

Calculate the pH of this buffer solution.

It says on the mark scheme that the number of moles of HCOOH is 0.24, but I don't understand why because I thought it would be the following:

moles of HCOOH= 3.2 X (200/1000)= 6.4 mol?

I think the final answer is 3.99 but don't understand...

Thank you in advance!
Original post by thesmallman
Hey! would be a reeaally big help if someone could explain to me this chemistry question on buffer solutions!

A chemist prepares a buffer solution by mixing together the following.

200cm^3 of 3.20moldm^-3 HCOOH (Ka=1.70X10^-4 moldm^-3) and 800cm^3 of 0.5 moldm^-3 NaOH.

The volume of the buffer solution is 1.00 dm^3

Calculate the pH of this buffer solution.

It says on the mark scheme that the number of moles of HCOOH is 0.24, but I don't understand why because I thought it would be the following:

moles of HCOOH= 3.2 X (200/1000)= 6.4 mol?

I think the final answer is 3.99 but don't understand...

Thank you in advance!


I've done this old OCR question before for a couple of my students. I'll just find the answer and post it in a minute or two.
I recognised the 3.99.

image.jpeg
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thesmallman
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Original post by TeachChemistry
I recognised the 3.99.

image.jpeg

Thanks a lot for your help!!!
I understand it now... it was a strange question tho :smile:

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