Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jokulsa on October 05, 2014, 11:00:02 AM
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1. How many moles of HF (pKa = 3.14) must be added to 500mL of 0.30M sodium fluoride (NaF) to give a buffer of pH 3.5? Ignore volume changes due to addition of hydrofluoric acid.
2. What is the molarity of CH3COO- in a buffer with a final pH of 5.20 if the concentration of CH3COOH is 0.25M? (pKa of CH3COOH is 4.7)
thanks
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Do you know Henderson Hasselbalch equation
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yeah I know it.
I think I managed to work out the second one, I don't really know where to start on the first..
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The first one is similar
develop HH-equation for it
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Is the 500mL given in the first question just there to confuse you?
If so, would the solution be
3.50 = 3.14 + log(0.30/[acid])
and then finding [acid] mathematically?
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Yes, but the value is wrong what does 0,3 M means
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Isn't this the [base]?
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M = molarity it has the unit mol/l, so how much mole you have in 500 ml
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I believe you would have 0.15 moles in 500mL
Why do you need to know this for this question?
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Write out the Henderson-Haselbach equation for us. Then write it again, with the correct values in the correct places. We'll look and see where you might be getting it wrong. That's how this forum works: http://www.chemicalforums.com/index.php?topic=65859.0
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I believe you would have 0.15 moles in 500mL
Why do you need to know this for this question?
You have HH equation you have pH, pka and the amount of the moles of salt, now develop the equation please.
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the HH equation is
pH = pKa + log([base]/[acid])
using the values I have,
3.50 = 3.14 + log([base]/[acid])
I'm sorry but I still don't understand where and how the moles of NaF come into it.
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What is the base and what is the acid here.
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HF is the acid and NaF is the base
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Yes correct. So now add the values into the HH equation.
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ok so is it
pH = pKa + log([base]/[acid])
3.50 = 3.14 + log([0.15]/[acid])
...and then finding [acid]?
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Yes correct. Solve the equation to Acid.
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I solved it and got 0.065M HF.
I used mol=M×vol to find the moles of acid
= 0.0325
Is this correct?
However, I still don't understand why I had to calculate the moles of the base, when the [base] of 0.30M was given in the question.
Would you be able to explain this please?
thanks
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The 0.065 mol is the result its not M. You convertet also the 0.3 M NaF to 0.15 mol
It was asks how much moles you have to add to 0.5 l. So for this reason you have to convert everything to the given volume.