1) You have available three materials, which are either a weak acid or weak base
ID: 795158 • Letter: 1
Question
1) You have available three materials, which are either a weak acid or weak base. The pKa values for the three materials are 3.64, 5.77, and 10.14. You have been hired by a law firm, Dewey, Cheatem, and Howe, as a consultant for a civil action against a chemical company accused of selling improperly prepared buffers. The litigant purchased 1000 liters of a pH 4.5 buffer, which was used in the manufacture of a pharmaceutical agent, Ipukalot. After using the buffer, the litigant found that the buffer capacity was much too low, and that the wrong material was used in the preparation of the buffer. Describe how you would prepare the buffer, assuming that the required buffer capacity (per 10 liters) must neutralize a minimum 5 mol of acid or 5 mol of base, and the pH change must not be greater than 0.5 pH units higher or lower. (You shouldnt have to make too many detailed calculations here, just remember that the buffer capacity is a minimum, and you can assume a relatively high solubility of whatever material you choose as your buffer)
Explanation / Answer
pH = pKa + log (salt / acid)
The main pH change is due to the change in the concentration of salt or acid.
Given the maximum of minimum change should be 0.5 means pH - pKa = log (salt/acid) = +- 0.5
Salt/Acid = 3.1622 (considering + sign) and 0.3162 (considering - sign)
When acid is added, salt/new acid = salt /(acid+5) >= 0.3162
For critical case, salt - 0.3162 acid - 1.582 = 0.....(1)
when base is added, base react with acid to form salt,
new salt/ new acid = (salt+5)/(acid - 5) <= 3.1622
salt - 3.1622 acid + 20.811 = 0 ......(2)
Solve, salt = 4.07 moles
acid = 7.86 mole
Add these quantities of salt and acid from the given weak acid and strong base.
You will get the required buffer.
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