Question

Predicting pH

Imagine you have a 0.125 M aqueous solution of aspirin, an acid drug with pKa = 3.5, in equilibrium. a) Estimate the pH of the solution Hint: Determine if it is an acid or base solution, weak or strong If weak acid/base, first try (H+) = /K,C, or [OH-] = K,C, b) Predict what would happen to the pH when you add more H2 Hint: First figure out what happens to the concentrations HA(aq) + H2O(0) A (aq) + H30+(aq) Use both kinetic and thermodynamic arguments.

Answer 1

a) Since the given pKa is 3.5, thus according to the pH scale we know that pKa<7 means acid and pKa>7 means basic solution, thus this pKa = 3.5 represents an acidic solution.

Given pKa = 3.5, thus we know that, $pKa = -logKa$

Thus the value of Ka (dissociation constant) will be, Ka = 10^-3.5 = 3.1622*10^-4

We know for weak acid, the expression for the concentration of protons in the solution, [H⁺] = $\sqrt{Ka\times Co}$

Where Co is the concentration of the solution = 0.125 M as given here.

Thus, the concentration of proton,

[H⁺] = $\sqrt{3.1622\times 10^{-4}\times 0.125}$ $\sqrt{3.1622\times 10^{-4}\times 0.125}= 6.287\times 10^{-3}M$.

Now, we know that pH = -log[H⁺] = $-log (6.287\times 10^{-3})=2.20$

$\therefore$pH of the solution is = 2.20

b) pH is the negative log value of the concentration of the proton. Thus, if the value of pH increase then it is a basic solution while the decrease in pH is an acidic solution. Now, here the reaction that happens after water is being added is:

$HA(aq)&plus;H_{2}O(l)\rightarrow A^{-}(aq)&plus;H_{3}O^{&plus;}(aq)$

If the concentration of water increases then the reaction will follow the Le Chatlier's principle and will move towards the forward direction to produce more amount of the H₃O+ ions and thus we can say that the value of the pH will decrease gradually and thus the solution will be more acidic after addition of water.

Another explanation is that, since, aspirin is a weak acid, thus on dilution with water, the number of the acid molecules will dissociate and thus the acidity will increase again for the same reason.