Themes > Science > Chemistry > Inorganic Chemistry > Acids and Bases > Acids and Bases Index > pH The acidity of a solution can be described by its concentration of H+. One way to express this concentration is in pH units, which stands for the Power of the Hydrogen ion concentration. pH is defined as pH = -log10[H+] (A similar term, pOH, can be used to describe basicity.) By the definition of pH, more acidic solutions will have a lower pH. If the pH of a solution is below 7, it is an acid, if it is above 7, it is basic. Why 7? Because water autoionizes: the concentration of H+ in pure water is 1.0*10-7 M, and thus the pH of pure water is -log(10-7) = 7. Example 1: Lemon juice has a H+ concentration of about 6.3*10-3 M, depending on the lemon. What is the pH of lemon juice? Solution: The pH is the -log10 of the hydrogen ion concentration, so pH = -log10(6.3*10-3) = 2.2 Example 2: The pH of household vinegar is about 3.0. What is the hydrogen ion concentration of vingear? Solution: Set up the equation as usual, then take the inverse log (10x on your calculator) of both sides to eliminate the log pH = -log10([H+]) 3.0 = -log10([H+]) -3.0 = log10([H+]) 10-3 = 10log10([H+]) = [H+] [H+] = 10-3 M Information provided by: http://learn.chem.vt.edu