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pKa confusion

Posted: Mon Jan 21, 2019 2:18 pm
by Katherine Grillo 1B
So when Ka is less than 10^-3, it is a weak acid right? So how does pKa work and what's the scale? If pKa is small, then is it a strong acid? And is it the same for pKb?

Re: pKa confusion

Posted: Mon Jan 21, 2019 2:39 pm
by Jayasuriya Senthilvelan 4I
Ka < 10^-3 is a weak acid. pKa = -log(Ka). So the higher Ka, stronger acid, lower pKa. Its like how if you have a really strong acid, the pH is super small. Same logic for pKb. Also the scale is 0 to 14 just like pH and pOH.

Re: pKa confusion

Posted: Mon Jan 21, 2019 2:56 pm
by Luc Lorain 1L
Strong acids (and bases) are assumed to dissociate almost completely in water, thus you can expect H3O+ formation to be strongly favored at equilibrium. As such, the product concentration in the equilibrium concentration formula should be expected to be much higher than the concentration of reactants, leading to a relatively large kA value.

The opposite is true for weak acids. These compounds are less likely to dissociate in water as they often do not possess as much ionic character as many strong acids. For weak acids, reactant concentrations remain larger than product formation leading to smaller equilibrium concentration values.