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Preparing a Buffer Solution

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Preparing a Buffer Solution

Postby Elinor » Sun Sep 07, 2008 12:12 am

Hi everyone,
I have a question about preparing buffer solutions. I've found some sites with some information on it but i'm having trouble understanding it and relating the info to my particular question.
I need to prepare 4 litres of o.4 mol mops (propane sulphonic acid) buffer of pH 7.8. (mops is just the name of the buffer).
In the lab I am given 10mols HCL and 10 mols NAOH and the zwitterionic form of mops which has a molecular weight of 210.3grams. The pKa for the mops zwitter ion is 7.2.

Now my question is...what exactly is the zwitter ionic form of mops...is that the conjugate base of the mops acid?
And if it is the conjugate base would you need to convert some of it to the acid to get the buffer?...because a buffer is a mixture of an acid and its conjugate base right?

HELP PLEASE!!!!!!!!!! :?
Elinor
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Re: Preparing a Buffer Solution

Postby blcr11 » Sun Sep 07, 2008 10:40 am

I’m not sure about some of your units. I would guess that you mean the free acid or the non-ionized form of MOPS when you call it the “zwitterionic form.” Then, when you say your solutions of HCl and NaOH are 10 mols, I don’t think that’s what you mean. I can only guess that the concentrations you intended are molarity (M for short). So your problem is to prepare 4 L of 0.4 M MOPS at pH 7.8 and you have the free-acid (MW of 210.3, pKa of 7.2) and 10 M solutions of HCl and NaOH, which you’ll use to adjust the pH of the solution.

In brief what you need to do is first calculate how much MOPS you need, and then add either HCl or NaOH to give you the final pH you want.

To calculate how much MOPS you need for 4 L of 0.4 mole/liter of the free-acid (what you call the “zwitter ionic” form) is straightforward.

4 L x 0.4 mole/L x 210.3 gm MOPS/mole = 336.5 g MOPS

This is a standard calculation to do in chemistry and biology. If you intend to continue on in these areas, you need to be able to do this kind of calculation pretty much on demand without having to think a lot about it.

Dissolve 336.5 g MOPS in 2-3 L of water. What pH do you think you have at this point? It won’t be 7.8. Most likely it will be acidic; just how acidic depends on how readily the free acid dissociates and how soluble the free acid is. (The very similar buffer, MES, for example, doesn’t even go into solution very well until you start to adjust the pH with NaOH.) You could calculate how much 10 M NaOH you’re going to need to get the pH of the solution to 7.8 by using the Henderson-Hasselbach equation. That’s a useful exercise, but not always practical. The easiest thing is to put your 2-3 liters of MOPS solution under a pH meter and add 10 M NaOH dropwise until you get to pH 7.8. If you should accidently overdo it, you can back-titrate with the 10 M solution of HCl. Once you have your final pH, dilute your pH-adjusted solution to a final volume of 4 L.
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