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I'm having a bit of trouble with this question:

Which has a higher $\mathrm{pH}$ value, $\pu{0.001 M}$ $\ce{HCl}$ or $\pu{0.1 M}$ $\ce{CH3COOH}$ $(K_\mathrm{a} = \pu{1.8E-5})?$

I have calculated that for the $\ce{HCl}$ solution (since it's a strong acid) the $[\ce{H3O+}] = \pu{0.001 M}.$

Then, for the $\ce{CH3COOH}$ solution I got $[\ce{H3O+}] = \pu{1.33E-3 M}$ (by solving a quadratic equation).

Since the $\ce{CH3COOH}$ has a higher concentration of hydronium ions, doesn't this mean that it will have a lower $\mathrm{pH}$ than the other solution? I'm confused because the answer says it's the other way around.

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    $\begingroup$ The concentrations look correct (next time please show your calculations and note that there is no zero in $\ce{H3O+}$), so you end up with pH 3.00 for HCl and pH 2.88 for acetic acid. What does the answer say, exactly? Could there be a typo? $\endgroup$
    – andselisk ♦
    Nov 9, 2019 at 7:10
  • $\begingroup$ The HCl solution has the higher pH value. The acetic acid solution has the higher concentration of hydrogen ions. What exactly did the answer key say? $\endgroup$
    – Karsten - apprentice ♦
    Nov 10, 2019 at 21:37

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