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Home Concept Builders Acids and Bases pH and pOH Help Help 3

pH and pOH - Questions 3 Help

A pH scale is used as an indicator of the hydronium ion (H3O+) concentration in an aqueous solution. And a pOH scale is used as an indicator of the hydroxide ion (OH-) concentration in an aqueous solution.

There are four tables for this difficulty level. Each table has the same basic structure with simply different numerical values. Here is one of the versions:
 

Version 1:

Express your understanding of pH, pOH, ion concentrations, and the identity of a solution as being acidic, basic, or neutral by completing the table.

 

Help for Wizard Difficulty Level

Water has a weak tendency to dissociate into ions - the hydronium ion (H3O+) and the hydroxide ion (OH-). The tendency of water to form these ions is so weak that the concentrations of ions in solution is very small. A pH scale is used as an indicator of the hydronium ion concentration. It is a logarithmic scale that is commonly used in science and math to express the magnitude of very large and very small numbers. Because the pH scale is a logarithmic scale, a low pH value is an indicator of a higher hydronium ion concentration. The equation that expresses the relationship between pH and hydronium ion (H3O+) is ...
 

pH = - log ([H3O+])


To determine the hydronium ion concentration ([H3O+]) from a pH value requires some algebra performed on the above equation. The result of the algebra manipulation is the following equation.
 

[H3O+] = 10-pH


A pOH scale is also used to indicate the concentration of the other ion - the hydroxide ion (OH-). The equations relating the pOH and the [OH-] are similar to those for the pH scale. They are ....
 

pOH = - log ([OH-])


[OH-]= 10-pOH


At a temperature of 25°C, the product of these two ion concentrations is approximately 1.0 x 10-14. This fact is the basis of the pH-pOH relationship, at least for temperatures of 25°C. The pH value and the pOH value add up to 14.

pH + pOH = 14 (at 25°C)


A neutral solution has the same pH value as its pOH value. So a pH of 7 corresponds to a neutral solution. Any pH value less than 7 corresponds to an acidic solution and any pH value greater than 7 corresponds to a basic solution.
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