Mixed micelle solutions were prepared by combining and diluting two detergent mi
ID: 485437 • Letter: M
Question
Mixed micelle solutions were prepared by combining and diluting two detergent micelle stock solutions (decyl maltoside and dodecyl maltoside each at 10% (w/v)) to a concentration of 0.5% (w/v) each in a final buffer consisting of 200 mM MES buffer, pH 5.5, and 150 mM NaCl.
1) Explain how you would prepare 925 mL this buffer at the target pH. Look up all molecular weights required and provide (in grams) how much of each component needs to be added, or if added from a concentrated stock, the required volume. What will the pH of the solution be if you initially dissolve the buffer in 1/2 the target volume (MES, pKA= 6.16)
2) If you added too much titrant and missed the desired pH by 1.0 units, what would you do to fix the mistake and still have a solution with the same composition? What should you avoid doing?
3) Determine the number of moles of the acid and its conjugate base at the desired pH.
4) Determine how many moles of weak acid and its conjugate base are present and the new pH if you were to add 3.0 mL of a 1.0 M solution of NaOH to 55 mL of the buffer.
Explanation / Answer
pH of a solution is dictated by Hinderson Hasselbalch equation
pH = pKa + log [conjugate base/weak acid]
5.5 = 6.16 + log [conjugate base/weak acid]
or, [conjugate base/weak acid] = 0.22
Calculate the decimal fraction of weak acid and conjugate base
[conjugate base] = 0.22/1+0.22 = 0.18
[weak acid] = 1/1+0.22 =0.82
Molarity of conjugate base = 0.18 * 200mM =36mM
Molarity of weak acid = 0.82 *200mM = 164 mM
moles of conjugate base = 36mM *0.925 L = 33.3 mmol = 0.0333mol
Mass of conjugate base requird= 0.0333mol * molar mass of the conjugate base =0.0333 mol * 218.2g/mol = 7.27g
Moles of weak acid = 164 mM * 0.925 L = 151.7 mmol = 0.1517 mol
mass of weak acid = 0.1517 mmol * 195.2 g/mol = 29.61 gm
(You have not mentioned what conjugate base of MES u r using. Generally a Sodium salt is used. So, I have done the calculations with sodium salt of MES)
Even if you add hlf the volume pH will be same as long as you keep the ratio of conjugate base and weak acid same.