The strengths of acids and bases are measured by the concentration levels of Hydrogen or Hydroxide ions. If a solution is neutral, the...

The "concentration" of a substance is indicated by placing square brackets around the substance.

• The concentration of `~H^+` ions is written [`~H^+` ]

• The concentration of `~OH^-` ions is written [`~OH^-` ]

If [`~H^+` ] is greater than [`~OH^-` ], the substance is an acid. Acids have a pH that is less than 7.

If [`~OH^-` ] is greater than [`~H^+` ], the substance is a base. Bases have a pH that is greater than 7.

If [`~H^+` ] is equal to [`~OH^-` ], the substance is neutral. Neutral substances have a pH = 7.

Pure water is neutral. Therefore, [`~H^+` ] is equal [`~OH^-` ]. The ionization constant (`~K_w` ) is equal to 1.0 x `~10^-^14` . `~K_w` can been described by the following formula:

   `~K_w` = [`~H^+` ][`~OH^-` ] = 1.0 x `~10^-^14`

Since, the `~H^+` and `~OH^-` concentrations must be equal, [`~H^+` ] and [`~OH^-` ] must be equal to 1.0 x `~10^-^7` . 

Therefore, the answer is c.