The pH scale is used to measure the acidity or alkalinity of a solution. The pH of a solution is defined as pH=-log [H], where [H] is the hydrogen ion concentration in moles per litre (mol/L). Solutions with a pH of less than 7 are acidic, and solutions with a pH of greater than 7 are basic, or alkaline. The lower the pH, the more acidic the solution.

a. A common ingredient in cola drinks is phosphoric acid, the same ingredient found in many rust removers. A cola drink has a pH of 2.5. Milk has a pH of 6.6. How many times as acidic as milk is a cola drink?

b. An apple is 5 times as acidic as a pear. If a pear has a pH of 3.8, what is the pH of an apple?