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/sci/ - Science & Math

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2110012 No.2110012 [Reply] [Original]

What is pKa? Every explanation I've found just goes into equations and very technical language. Can someone explain it in layman's terms?

>> No.2110023

It allows you to grade strengths of acids.

>> No.2110033

>>2110023

I don't expect anyone to do my homework, but I'm supposed to work out the pKA of a substance from a graph of absorbance against pH. Apparently it's the value of x when y is half the maximum value, and I've got no idea why.

>> No.2110055

basics of electrochemistry.

consider the reaction (at equilibrium of course):

HA --> A− + H+

the dissociation constant (of the reaction) is written by:

Ka = [A-][H+]/[HA]

but the problem here is that the constant goes from fractions of decimal places to millions (many many orders of magnitude). So what do we do to simplify it? use logarithms!

Therefore:

pKa = log10(Ka). (pKa is just the acid dissociation constant)

weak acids are generally in the range -2 to 12. less than -2 is generally considered strong acids.

>> No.2110069

>>2110055
sorry,
its actually
pKa = -log10(Ka)

>> No.2110107

>>2110055

> -2

Negative 2, or just a hyphen? I've never come across negative pH before.

>> No.2110130

I remember this shit.. That is to say, I vaguely remember having learned this shit, though I no longer know it lol. Fuck I'm dumb.

>> No.2110133

>>2110107
You should take the negative of the aforementioned equation.

>> No.2110147

>>2110107

note that its pKa = -2 to 12 which is a weak acid (yes negative here)

pKa = log10(Ka)

pH = -log10[(Ka*c0)^1/2]

where Ka= [A-][H+]/[HA] and c0 is the concentration of the acid.

acid dissociation constant (pKa) is NOT EQUAL to power of hydrogen (acidity or alkalinity, pH)

>> No.2110179

Thanks for all of your help.

It's 1am here, so I'm pretty tired and my mind's blank. I can't find anywhere on the internet that actually explains what [A] and [HA] are. I know that [H+] is proton concentration, and I'm guessing that [A] is the anionic product of [HA], where [HA] is the molecule that dissociates to form a cation and an anion?

>> No.2110244

>>2110179

correct