Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: ajones51 on October 08, 2006, 12:50:18 PM
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Hi everyone,
I have been trying to solve these two questions but am having trouble doing so and was wondering if someone could help me out.
(1) You wish to make a 0.200 mol/L PIPES buffer (PIPES:
1,4-Piperazine-N,N'-bis(2-ethane-sulfonic acid), C8H18N2O6S2, pKa = 6.80) at a
pH of 6.300. Available to you is the disodium salt of PIPES (Na2C8H16N2O6S2),
and a 2.00 F solution of HCl. Calculate
i)the mass of PIPES,
ii) the volume of HCl,
that must be added to make 500.0 mL of this buffer.
I tried to initally use the Henderson Hasselbach equation to solve for the ratio of weak base and its conjuate acid that would be needed to make the buffer with the desired pH but this only gets me so far. As HCL must be added i figured that I would try setting the # of moles of base in the H.Hasselbach equation to .200 + x since more moles of base will be needed to help counteract the fact that HCl is being added. However, I end up with an equation that just gives me back the original ratio of weak base and its conjugate acid, which is not correct. I have also tried writing out an acid base equation in an attempt to see what is happening with the number of moles, but I am ending up nowhere. If someone out there could please give me a hand or show me some pointers on how to solve this problem, that would be greatly appreciated.
2.
A chemist is following a hydrolysis reaction with an enzyme. The enzyme has an
optimal working pH range of 7.40±0.05. It is known that H+ is evolved during
the hydrolysis reaction. The chemist does a quick calculation and determines
that the reaction she is following could generate up to 0.001 moles of H+ per
100.0 mL of solution. Describe how to make up 500.0 mL of the HEPES buffer
(HEPES: 4-(2-Hydroxyethyl)piperazine-1-ethanesulfonic acid, C8H18N2O4S, pKa =
7.55) such that the reaction mixture will maintain a pH that is within the
optimal working range of the enzyme. Among the reagents in the lab are HEPES as
the free acid form, and a 5.695 F solution of NaOH. What mass of HEPES and what volume of NaOH should you use?
Note: while several solutions are possible, here you are asked to calculate the
minimum amount of HEPES that could still maintain the pH within the desired
limits.
I have tried to set up an acid base reaction to solve for the number of moles of acid are being created, but when I solve for the pH, it ends up being outside the range of the enzyme, and while I realize that one should try and minimize the amount of Hepes being used, I can't set up a mathetmatical equation that will account for this, without thinking of derivatives and setting them equal to zero to find the minimum. Could someone please give me a bit of help?
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and a 2.00 F solution of HCl. Calculate
What does F stand for? Did you mean M?
i)the mass of PIPES,
Are you sure? Isn't it the mass of the disodium salt of PIPES?
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F means Formal...Old expression for solutions
chiralic