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8.4 The pH scale
Terms in this set (10)
Describe the pH scale
The pH scale can be used to indicate whether a solution is acidic, alkaline or neutral. At 25°C, a solution with pH lower than 7 is acidic, a solution with pH 7 is neutral, and a solution with pH greater than 7 is alkaline.
The pH of a solution can be determined using a pH meter or by using universal indicator solution or paper.
pH is a measure of the concentration of H+(aq) ions in a solution. The concentration of H+ ions can be stated in mol dm(-3), but the use of the log(10) function simplifies the numbers involved.
Definition: pH is the negative logarithm to base 10 of the hydrogen ion concentration in aqueous solution.
pH = -log(10)[H+(aq)]
pH has no units.
State the relationship between pH and the H+ concentration
Because pH is a log scale (to base 10), a 1 unit change in pH indicates a tenfold change in the H+ ion concentration.
Calculating [H+(aq)] from a pH
To calculate [H+(aq)] from a pH, the inverse function of log(10) must be used. So 10 must be raised to the power of -pH.
[H+(aq)] = 10^(-pH)
Describe the dissociation of water
Water dissociates (ionises) according to the equation:
H2O(l) ⇌ H+(aq) + OH-(aq)
The degree of dissociation is very small and, at 25°C, in pure water the concentrations of H+ and OH- ions are equal at 1.0 x 10^(-7) mol dm-3.
Define the equilibrium constant, K(w) (also called the ionic product constant)
K(w) = [H+(aq)][OH-(aq)]
K(w) is called the ionic product constant (ionic product) for water.
K(w) has a value of 1.0 x 10^(-14) at 298 K.
K(w) refers to the H2O(l) ⇌ H+(aq) + OH-(aq) equilibrium in all aqueous solutions - the product of the H+ and OH- concentrations in any aqueous solution at 298 K is 1.0 x 10^(-14).
Compare the values of [H+(aq)] and [OH-(aq)] or acidic, neutral and alkaline solutions
A solution is:
- Neutral if [H+(aq)] = [OH-(aq)]
- Acidic if [H+(aq)] > [OH-(aq)]
- Alkaline if [H+(aq)] < [OH-(aq)]
Calculating the pH of a solution of a strong acid
Because strong acids can be assumed to dissociate completely in aqueous solution, the concentration of H+ ions is the same as the concentration of hydrogen atoms in the acid.
1) Use the concentration of the acid to deduce the concentration of H+.
2) Use pH = -log(10)[H+(aq)] to find the pH
Calculating the pH of a solution of a strong base
Strong bases are completely ionised in solution.
The pH of an alkaline solution can be worked out using the ionic product constant, K(w), equal to 1.0 x 10^(-14) at 25°C.
1) Use the concentration of the alkali to deduce [OH-(aq)]
2) Substitute [OH-(aq)] and K(w) = 1.0 x 10^(-14) into the expression: K(w) = [H+(aq)][OH-(aq)]
3) Find [H+(aq)]
4) Plot the value from 3) into pH = -log(10)[H+(aq)]
Nature of science: Occam's razor
The pH scale is an attempt to scale the relative acidity over a wide range of H+ concentrations into a very simple number. It is important that scientists are able to communicate their findings and discuss issues with non-scientists. pH is an example of a concept that allows discussion of acidity without the requirement for a complete understanding of the theory .
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