Wize High School Grade 12 Chemistry Textbook > Acids and Bases

pH equations and Problems

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pH Scale
The pH scale is a logarithmic scale used to measure the concentration of [H+] in a solution, since this value can vary over many orders of magnitude. It ranges from 0 to 14, with 7 representing neutral pH

Note: There are some exceptions to this. For example, an acid could have a negative pH! These aren't commonly seen. 

Photo by OpenStax College / CC BY

A lower pH indicates that a substance is more (acidic/basic) acidic
 and has a (higher/lower) higher 
 [H+] 
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We also have Ka and pKa equations: 

pKa=−log⁡⁡Ka and Ka=10−pKapK_a=-\log⁡K_a\ and\ K_a=10^{-pK_a}pKa​=−log⁡Ka​ and Ka​=10−pKa​

As well as Kb and pKb equations:

pKb=−log⁡⁡Kb and Kb=10−pKbpK_b=-\log⁡K_b\ and\ K_b=10^{-pK_b}pKb​=−log⁡Kb​ and Kb​=10−pKb​

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What does it mean to be a logarithmic scale? 
How does the [H+] change as the pH goes from 1 to 2 or 1 to 3?? 

Wize Concept
Since pH is a logarithmic scale, each 1 unit change in pH corresponds to a 10-fold change in [H+]!

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Example: pH Problem
If 0.2 mols of HCl(s)\text{HCl}_{(s)}HCl(s)​ are dissolved in 1.8 L of water, what is the pH? 
HCl → H+ + Cl-

Since HCl\text{HCl}HCl is a strong acid, we will be adding 0.2 mols of H+\text{H}^+H+ to 1.8 L of water.

[H+]=0.2mol1.8L=0.11mol/L[\text{H}^+]=\dfrac{0.2mol}{1.8L}=0.11mol/L[H+]=1.8L0.2mol​=0.11mol/L

pH=−log⁡[H+]=0.95pH=-\log[\text{H}^+]=0.95pH=−log[H+]=0.95

Practice: Solve for [H+]
A solution of HBr has a pOH of 12.2, what is the [H+] concentration?