Question #ab4a2

The pH of a solution is nothing more than a measure of its concentration of hydrogen ions, #"H"^(+)#, which you'll sometimes see referred to as hydronium cations, #"H"_3"O"^(+)#.

More specifically, the pH of a solution is calculated by taking the negative log base #10# of the concentration of hydrogen ions

#color(blue)(|bar(ul(color(white)(a/a)"pH" = - log(["H"^(+)])color(white)(a/a)|)))#

The problem already provides you with the concentration of hydrogen ions, so plug this into the equation to find

#"pH" = - log(1.7 * 10^(-5)) = color(green)(|bar(ul(color(white)(a/a)4.77color(white)(a/a)|)))#

Notice that because the pH is calculated using a negative log, a higher concentration of hydrogen ions will result in a lower pH.

In fact, the pH of pure water at room temperature is equal to #7# because pure water has

#["H"^(+)]_"pure water" = 1.0 * 10^(-7)"M" " "# and #" "["OH"^(-)]_"pure water" = 1.0 * 10^(-7)"M"#

Your solution has a higher concentration of hydrogen ions, which is why the pH if lower than #7#. This corresponds to an acidic solution.