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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
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