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Naked Science Forum
Non Life Sciences
Chemistry
Is pKa logarithmic?
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Is pKa logarithmic?
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Kryptid
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Is pKa logarithmic?
«
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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".
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Ylide
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Re: Is pKa logarithmic?
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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.
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