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How do the numbers on the pH scale compare? Example – is a pH of 4 twice as strong as a pH of 2? Remember the pH scale is not linear! Also, "is 4 twice of 2" isn't my question. I just want to know the way the numbers compare to each other.

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    $\begingroup$ They don't. There is no such thing as "twice as strong" among acids. $\endgroup$ Jun 22, 2017 at 19:43
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    $\begingroup$ en.wikipedia.org/wiki/PH $\endgroup$
    – jerepierre
    Jun 22, 2017 at 19:46
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    $\begingroup$ Ever heard about logarithms? $\endgroup$
    – Mithoron
    Jun 22, 2017 at 19:48
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    $\begingroup$ Perhaps it's better for you to ask your own question in your own words, instead of (what seems to be) copy-pasting somebody else's question here. $\endgroup$ Jun 22, 2017 at 22:12
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    $\begingroup$ I'm voting to close this question as off-topic because it is something that is typically taught in schools the minute the pH scale is introduced. $\endgroup$
    – Jan
    Jun 23, 2017 at 0:44

3 Answers 3

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$\mathrm{pH} = -\mathrm{log}[\mathrm{H}^+]$

i.e. the pH stands for the negative log (base 10) of the concentration of H$^+$.

this means in a solution of HCl (0.1 mol dm$^{-3}$ ) where the HCl is fully dissociated, i.e. $\ce{HCl -> H+ + Cl-}$ the concentration of H$^+$ is 0.1 mol dm$^{-3}$, so $\mathrm{pH} = -\mathrm{log}(0.1) = 1$

if the concentration of $\ce{H+}$ is 10 times greater, i.e. 1 mol dm$^{-3}$, so $\mathrm{pH} = -\mathrm{log}(1) = 0$

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The pH scale tells you how acid is a solution, or more formally the concentration of hydronium ion ($\ce{H3O+}$) you have in the solution. Remember that

$pH = - \log([\ce{H3O+}])$

therefore the concentration of $[\ce{H3O+}]$ in a solution with $pH = 4$ will be two orders of magnitude lower than the one with $pH = 2$.

If you do the $Antilog$ of -2 and -4 you will have $\pu{1E-2}$ and $\pu{1E-4}$ respectively.

So the higher the pH the less concentration of $[\ce{H3O+}]$ and less acid (or more alkaline).

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The pH is a measure of: $$\mathrm{pH} = -\mathrm{log}[\mathrm{H}^+]$$ It's a logarithm, for being more exactly it's a common logarithm (his base is 10).
In other worlds, this means that each pH is 10 times less "powerful" or acid that the last (it's one order of magnitude lesser).

For example:

  • $\mathrm{pH}=\mathrm{4}$ is 10 times more acid than $\mathrm{pH}=\mathrm{5}$, or it's one order of magnitude (x10 times) higher.

It can be some strange to think, but, higher numbers are not more acids, it's the opposite, higher numbers are less acids and lower numbers are more acids.

I hope this could help.

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  • $\begingroup$ Why the down-vote? $\endgroup$
    – Ender Look
    Jun 23, 2017 at 15:00
  • $\begingroup$ Base 10 logs are called "common logs". Base $e$ logs are the "natural logs". $\endgroup$ Jun 24, 2017 at 0:41
  • $\begingroup$ @PrittBalagopal Ups, sorry, I'll fix that. $\endgroup$
    – Ender Look
    Jun 24, 2017 at 2:20
  • $\begingroup$ And I shall remove my downvote :) $\endgroup$ Jun 24, 2017 at 2:57

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