# The Naked Scientists Forum

### Author Topic: Is pKa logarithmic?  (Read 10757 times)

#### Supercryptid

• Hero Member
• Posts: 606
##### Is pKa logarithmic?
« on: 10/12/2003 19:03:04 »
pKa is the measurement of how acidic a molecule is (how easily it gives up a proton). The lower the pKa of a molecule, the more acidic it is.

But I have a question; is the pKa scale logarthmic? If it is, then Hydrogen Iodide (pKa = -10) should be about 10 times more acidic than Hydrogen Bromide (pKa = -9). If the scale is not logarithmic, then HI should only be about 1.1 times more acidic than HBr. So is the scale logarithmic or not?

Oh yeah, and another question; what is the pKa of Hydrogen (H2)? My college textbook just says that it is "very large".

#### Ylide

• Moderator
• Hero Member
• Posts: 905
##### Re: Is pKa logarithmic?
« Reply #1 on: 10/12/2003 19:17:30 »
Yes, pKa is logarithmic.  It's mathematically identical to pH, where one whole integer of difference is a factor of 10.  The Ka (acid dissociate constant) is calculated by 10^-pKa.  So, if the pKa is a negative number, the dissociation constant will be quite large, therefore the H+ ion concentration quite large when the acid is dissolved in water, thus making it a stronger acid.  Conversely, a very large pKa means a very small Ka and very little H+ dissociation.

I don't have an exact number for the pKa of H2, but now you know what a "very large" pKa means.  Neutral organic compounds have pKa's on the order of 40-60, I would imagine hydrogen being in that ballpark...in other words, billions of times less acidic than real acids.

This message brought to you by The Council of People Who Are Sick of Seeing More People

#### The Naked Scientists Forum

##### Re: Is pKa logarithmic?
« Reply #1 on: 10/12/2003 19:17:30 »