The pH scale is a way of expressing the strength of acids and bases. Solutions can be further characterized by their concentrations of hydrogen ion or hydroxide ion in water. Brackets ([ ]) around a compound are used to indicate “concentration” (Molarity). pH = - log [H + ]. The stronger the acid, the lower the pH. Typically, the pH of a substance will be between 0 and 14. pH < 7 → acidic solution pH = 7 → neutral solution pH > 7 → basic solution • If the [H + ] is greater than 1 × 10 -7 M, the solution is considered acidic, and the resulting pH will be less than 7. If [H + ] less than 1 × 10 -7 M, it is considered basic, and the resulting pH will be greater than 7. If the [H + ] is equal to 1 × 10 -7 M, the solution is considered neutral, and the resulting pH will be equal to 7. • If the [OH - ] is greater than 1 × 10 -7 M, the solution is considered basic, and the pH will be greater than 7. If the [OH - ] is less than 1 × 10 -7 M, it is considered acidic, and the resulting pH will be less than 7. If the [OH - ] is equal to 1 × 10 -7 M, the solution is considered neutral, and the resulting pH will be equal to 7. Example 1 : Based on the concentrations of the H + /OH - provided, are the following solutions acidic or basic? a) [OH - ] = 1 x 10 -5 M Answer: (basic) b) [H + ] = 1 x 10 -10 M Answer: (basic) c) [H + ] = 1.0 M Answer: (acidic) Reporting concentration values in terms of molarity is not always very useful because of the considerably small concentrations that are dealt with. A scale, known as the pH scale , was created to more easily communicate the concentration of acids or bases. The pH is defined as pH = -log[H + ] . Example 2: What is the pH of a solution that has [H + ] = 1 x 10 -6 M? pH = -log[1 x 10 -6 ] = 6 Example 3: What is the pH of the solution when [H + ] = 1 x 10 -11 M? pH = -log[1 x 10 -11 ] = 11
You may notice that the pH is the related to the exponent in both of these examples. That only holds true if the number in front of the exponent is 1. If it is some other number, such as 6.2 x 10 -5 M, a calculator must be used to determine the pH. Also notice that there are no units for pH. pH Scale The pH scale can be confusing at first because HIGHER hydrogen (H + ) concentrations correspond to LOWER pH values. LOWER hydrogen ion concentrations correspond to HIGHER pH values. The pH scale is often show as ranging from pH=0 to pH=14, though higher and lower values can be obtained. Most acids and bases solutions fall within 0 to 14, though. pH values lower than 7 indicate acidic solutions, while pH values higher than 7 indicate basic solutions. pH=7 is known as neutral. Pure water would be neutral. Standardization: For the standardization process, a method must be used which can be expected to give accurate results. Since sodium hydroxide is a base, it can be reacted with a known amount of acid in a neutralization reaction. An accurately calibrated piece of glassware known as a burette is used to keep account of the volume of base used to react with the acid. If the acid can be weighed accurately, then the sodium hydroxide concentration can be reliably determined from the volume of base and the mass of acid used in the reaction.
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