What does it mean if a solution has a pH of 6.0?

1 Answer
Oct 23, 2017

Answer:

That it contains #10# times more hydronium cations than a neutral solution at room temperature.

Explanation:

As you know, the #"pH"# of a solution is a measure of the concentration of hydronium cations, #"H"_3"O"^(+)#, present in this solution.

More specifically, to find the #"pH"# of a solution, you need to take the negative log base #10# of the concentration of hydronium cations.

#"pH" = - log(["H"_3"O"^(+)])#

You can rewrite this equation as

#["H"_3"O"^(+)] = 10^(-"pH")#

Now, pure water at room temperature has

#"pH" = 7#

This implies that the concentration of hydronium cations in pure water at room temperature is equal to

#["H"_3"O"^(+)] = 10^(-7)color(white)(.)"M"#

#["H"_3"O"^(+)] = 1 * 10^(-7)color(white)(.)"M"#

In order for the #"pH"# of the solution to decrease by #1# unit, the concentration of hydronium cations must increase by an order of magnitude, i.e. #10#-fold.

So for #"pH" = 6.0#, you have

#["H"_3"O"^(+)] = 10^(-6.0)color(white)(.)"M"#

#["H"_3"O"^(+)] = 1 * 10^(-6)color(white)(.)"M"#

This corresponds to an increase by an order of magnitude in the concentration of hydronium cations, since

#(["H"_ 3"O"^(+)]_ "pH = 6.0")/(["H"_ 3"O"^(+)]_ "pH = 7") = (1 * 10^(-6)color(red)(cancel(color(black)("M"))))/(1 * 10^(-7)color(red)(cancel(color(black)("M")))) = 10#

You can thus say a solution that has a #"pH"# equal to #6.0# contains #10# times more hydronium cations than a solution that has #"pH" = 7#, i.e. than a neutral solution at room temperature.

This, of course, implies that you are dealing with a solution that is slightly acidic, since that is what #"pH" < 7# at room temperature implies.

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