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We could describe the relative strengths of dilute solutions of acids and bases by listing the molarity of H+ for acidic solutions and the molarity of OH- for basic solutions. There are two reasons why we use the pH scale instead. The first reason is that instead of describing acidic solutions with [H+] and basic solutions with [OH-], chemists prefer to have one scale for describing both acidic and basic solutions. Because the product of the H+ and OH- concentrations in such solutions is always 1.01 × 10-14 at 25 °C, when we give the concentration of H+, we are indirectly also giving the concentration of OH-. For example, when we say that the concentration of H+ in an acidic solution at 25 °C is 10-3 M, we are indirectly saying that the concentration of OH- in this same solution is 10-11 M. When we say that the concentration of H+ in a basic solution at 25 °C is 10-10 M, we are indirectly saying that the OH- concentration is 10-4 M. The pH concept makes use of this relationship to describe both dilute acid and dilute base solutions on a single scale.
The next reason for using the pH scale instead of H+ and OH- concentrations is that in dilute solutions, the concentration of H+ is small, leading to the inconvenience of measurements with many decimal places, such as 0.000001 M H+, or to the potential confusion associated with scientific notation, as with 1 × 10-6 M H+. In order to avoid such inconvenience and possible confusion, pH is defined as the negative logarithm of the H+ concentration.
pH = -log[H+]
Instead of saying that a solution is 0.0000010 M H+ (or 1.0 × 10-6 M H+) and 0.000000010 M OH- (or 1.0 × 10-8 M OH-), we can indirectly convey the same information by saying that the pH is 6.00.
pH = -log[H+] = -log(1.0 × 10-6) = 6.00
When taking the logarithm of a number, report the same number of decimal positions in the answer as you had significant figures in the original value. Because 1.0 × 10-6 has two significant figures, we report 6.00 as the pH for a solution with 1.0 × 10-6 M H+. The table below shows a range of pH values for dilute solutions of acid and base.Unsaturated fatty acids have one or more double bonds between carbon atoms. (Pairs of carbon atoms connected by double bonds can be saturated by adding hydrogen atoms to them, converting the double bonds to single bonds. Therefore, the double bonds are called unsaturated.)
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