PH and buffers

  • Thread starter jsmith613
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  • #1
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Homework Statement


An buffer is made from a 0.1M acid and a 0.25 M salt.
According to the equation
pH = pKa - log[acid]/[base]
the ratio acid:base is 1:2

What volumes are required to make a 1 dm3 solution?

Homework Equations





The Attempt at a Solution


According to the 1:2 ratio we would need
333.3 cm3 of acid and 666.6 cm3 of base
However as the initial concentration of acid to base is 0.1:0.25 we instead say
volume acid = 333.3cm3
volume base = 666.6 cm3/2.5 = 266.6 cm3
volume water = 400 cm3

(all of these volumes are rounded to the nearest 0.1 cm3)

is this correct?
 
Last edited:

Answers and Replies

  • #2
Borek
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Question doesn't say anything about using additional water for dilution, so I don't think that's the correct answer.
 
  • #3
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yes but as I need to make up a litre the rest of the solution must be water
 
  • #4
Borek
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No, you don't have to add water. If you mix correct volumes of both solutions you will have a liter of solution with a correct ratio of acid and conjugate base. Try to write two equations describing both ratio and volume - that will give you two equations in two unknowns, easy to solve.
 
  • #5
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666.67 cm3 acid
and
333.333 cm3[/base]
 
  • #6
Borek
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Have you checked if the ratio is correct if you use these volumes?
 
  • #7
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ratio acid:base assuming 1M:1M initial ratio gives 1:2
BUT initial molar ratio is 1:2.5
therefore new acid:base ratio from pH = pKa - log[acid]/[base] is 1:(2/2.5) = 1:0.8
...oh
acid: 555.6 cm3
base: 444.4 cm3

right?
 
  • #8
Borek
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Sorry, we are not checking homework, we are helping solve it.
 
  • #9
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This is not homework
this is a revision question I can't find an answer to
am i right?
 

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