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# Ka and pKa review

A short review of Ka and pKa for students taking organic chemistry.

## Want to join the conversation?

• I guess the "a" on Ka and pKa means "acid", but what does "K" on Ka and "pK" on pKa stand for?
(1 vote)
• I haven't been able to find any material to definitively say exactly why, but my best guess is that the 'K' stands for constant. The scientists which popularized this idea of chemical equilibrium were Norwegian chemists Maximilian Guldberg and Peter Waage. Constant in Norwegian is spelled "konstant", starting with a 'K'. So the 'K' in equilibrium equations (and also kinetics) could just be constant in Norwegian.

Alternatively Guldberg and Waage also published papers in Danish and German where constant is spelled "konstant" and "konstante" respectively. So the 'K' could also mean constant, but in either of those languages too.

Again I'm not 100% sure that's the origin, but it's the most likely origin in my opinion.

Hope that helps.
• Why does making the log negative mean the number is easier to work with? Thanks
(1 vote)
• Almost all of the acids you work with in organic chemistry have a Ka that takes the form a*10^-b, where a is a positive real number and b is a non-negative integer. As such, almost all of the pKa values will be positive. If you don't take the negative log and instead take the positive log, then you'll be working with negative numbers constantly. Making the value negative doesn't really distort the meaning of the final number; it just changes the direction and switches the sign. Instead of having lots of negative numbers where the lower values are stronger acids, you have lots of positive numbers where the higher values are weaker acids.
• what is the physical significance of Ka and pKa ... that is what each means?

what the use of pKa
(1 vote)
• Ka is defined by the equation given, [H+][A-]/[HA]. In general, you can kind of sort of think of equilibrium constants as the ratio between the products and the reactants (you have to be careful to include any coefficients and to check the states of matter when dealing with some kinds of reactions, but this is close enough to develop loose intuition). Since equilibrium is a situation where the reaction goes both ways, looking at the equilibrium constant gives us an idea of which way the reaction goes more. When it is larger than one, more products are formed than reactants, so the reaction mostly goes forward. When it is smaller than one the opposite is true (note that for weak acids Ka is much smaller than one, which means that, while some of the acid dissociates, most of it stays together).

pKa is simply negative one times log base 10 of Ka. This is done just to make the numbers easier to work with, the same way we tend to talk about acidity in terms of pH rather than [H+].
(1 vote)
• Would the pattern of weak acid strength in relation to Ka/pKa values directly correspond to the strength of weak bases and their respective Kb/Pkb values?
(1 vote)
• how did you get those numbers like HF= 3.5 x 10? how do i know for example [ag+][CI-] number would it be? is there a rule for each that would have numbers written for them? I dont understand that part from where do those number appear 😅
(1 vote)
• Look up “table of Ka values” on Google. If you have a chemistry textbook there’s probably a table with a bunch of them too.

AgCl will be a Ksp one...totally different from this videos topic....
(1 vote)
• is Ka always given ? if not how do solve with just the equation with the product over the reactant
(1 vote)
• If you have to calculate concentrations, the Kₐ will be given.
Sometimes they will give you concentrations, and you will have to calculate the Kₐ.
(1 vote)
• What is the relationship between pKa and side chain charge?
(1 vote)
• Unfortunately, there's no general relationship between these two.