# Hydronium ion concentration to compare pH values

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.

• 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? Nov 9, 2019 at 7:10
• 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? Nov 10, 2019 at 21:37