Which of the following has the greatest concentration of hydrogen ions? How is this determined?

pH is a shorthand numerical value that represents the concentration of hydrogen ions in solution.

The scale 0-14 represents the usual range of possible concentrations of protons (#H^+#) in an aqueous system.

Possiamo considerare #H^+# to mean the same as #H_3O^+# (a water molecule carries
il #H^+#).

#"pH"# is the negative log of hydrogen ion concentration, meaning that:

#"pH" = -log[H_3O^+] = -log[H^+]#

and therefore:

#10^-"pH" = [H^+]#
#10^(-3) = 1.0 × 10^(-3)color(white)(l) "mol/L"#

Let's simplify:

#"pH"color(white)(m) [H^+] ("mol/L")#
#color(white)(ll)3color(white)(mml)1 × 10^(-3)#
#color(white)(ll)5color(white)(mml)1 × 10^(-5)#
#color(white)(ll)7color(white)(mml)1 × 10^(-7)#
#color(white)(ll)9color(white)(mml)1 × 10^(-9)#
#color(white)(l)11color(white)(mm)1 × 10^(-11)#

This is counter-intuitive without the background information (i.e. the negative log) but, as you increase pH, you decrease the concentration of #H^+#.

As you decrease the pH, you increase the concentration of #H^+#.