Map sapling learning Strong base is dissolved in 525 mL of 0.200 M weak acid (Ka

Question

Map sapling learning Strong base is dissolved in 525 mL of 0.200 M weak acid (Ka J 3.90 x105) to make a buffer with a pH of 3.97. Assume that the volume remains constant when the base is added HA (aq)+ OH (aq) H,o (l) A (aq) Calculate the pKa value of the acid and determine the number of moles of acid initially present. Number Number p mol HA When the reaction is complete, what is the concentration ratio of conjugate base to acid? Number HA How many moles of strong base were initially added? Number mol OH

Explanation / Answer

To get the pKA we need to use the following expression:
pKa = -logKa
pKa = -log(3.9x10-5) = 4.41

the inital moles of HA are:
moles HA = 0.2 * 0.525 = 0.105 moles

the ratio would be:
pH = pKa + logA/HA
3.97 = 4.41 + logA/HA
3.97-4.41 = logA/HA
A/HA = 10-0.44
A/HA = 0.3631

Finally to get the moles of base initially added, let's calculate the moles of A-
A/HA = 0.3631
A = 0.3631*HA
A = 0.3631 * 0.105 = 0.038 moles

The reaction was:
HA + OH- --------> A- H2O
so a 1:1 relation will give us the mole of OH-

it would be 0.038 moles

Hope this helps

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