TEAS Acid and Base Solutions
suppose the activity of hydrogen ions in a solution is 1 part per ten thousands. Find the pH value of the solution. Is this a basic or acidic solution?
pH = 4; Acid pH = -log (aH) = -log (1 X 10^-4) = -(-4) = 4
What does a difference of one unit on the pH scale below present?
A concentration difference of 10 A difference of one unit on the pH scale represents a difference of 10 in strength of concentration based on log10 (H+) or log 10 (OH-)
Acidic solutions (solutions with a pH value less than 7)
The pH scale is used to measure the strength of acidic solutions and the strength of basic solutions (solutions with a pH value greater than 7). Pure H2O at 25^o Celcius is recognized as the standard of neutrality between acids and bases because it has a pH value of 7.0. Stronger acids will have smaller numerical values on the pH scale than weaker acids, and stronger bases will have greater numerical values than weaker bases. Some sample values are illustrated in Table 3.7.
Numerical Value
Since activity (aH) measures concentrations of hydrogen ions, the numerical value is typically found to be much less than 1. In addition, since a larger number of hydrogen ions is associated with an acid than a base, the avtivity value for acids will be larger for acids than bases.
Regarding bases, a ____ number of hydrogen ions is associated with a basic solution (as opposed to an acidic solution), a red litmus paper will turn ____ in the presence of a base.
Smaller; Blue The calculation of activity requires a smaller percentage of hydrogen ions to yield values larger than seven, and litmus turns blue for bases.
Example 3.18
Suppose the activity of hydrogen ions in solution is 1 part per (i.e., 1/1,000,000,000 or 0.000000001). In scientific notation, this number equals 1 X 10^-9. To calculate the pH value, do the following: pH = - log (aH) pH = - log (1X10^-9) pH = -(-9) pH = 9 This value would then be considered to represent a weak base. Hand soap has a similar pH value.
Example 3.17:
Suppose the activity of hydrogen ions in solution is 1 part per thousand (I.e., 1/1,000 or 0.001). In scientific notation, thus number equals 1X10^-3. To calculate the pH value, do the following: pH=-log (aH) pH=- log (1X10^-3) pH=-(-3) pH=3 Thus value would then be considered to represent an acid of similar strength to orange juice, as seen from the previous table.
Calculation of pH:
The calculation of pH is based on the activity (aH) of hydrogen ions (H+) dissolved in solution. Specifically, the pH range of numbers is based on a logarithmic scale: pH=-log (aH) Log functions are used to rescale very large numbers (or very small numbers) into values much closer to 1. This is often done to deal with more reasonable Numbers (for pH, 1 to 14). Mathematically, the negative sign in the formula means that aH must be less than 1, seen in Examples 3.17 and 3.18.
pH Indicator:
If activity values are not readily known, a pH indicator is used to qualitatively determine the pH. A known weak acid or base (such as litmus paper) changes color when introduced to an unknown acid or base. Litmus paper will indicate red for acid solutions and blue for basic solutions. Even though pH indicators will not give precise measurements of pH, they are usually sufficient when only an approximate value is needed.