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Module 1: Acid, Bases and Salts

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Example Problems on Acid-Base Neutralization

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This video shows the step-by-step method for calculating pH and pOH.
pH is the measure of hydrogen ion [H+] concentration. pH is used as a numeric scale used to determine whether a solution is acidic, basic or neutral. pH has no units.
The pH of a solution is expressed as the negative logarithm of hydrogen ion concentration.
For example, given the hydrogen ion concentration of 3.1 X 10-4 Molar or moles/liter, what is the pH of the solution?
pH is equal to the negative log of hydrogen ion concentration.
Here, the concentration of hydrogen ions is 3.1 X 10-4 So, we substitute this value into the equation.
The pH is equal to negative log of 3.1 X 10-4. Now using the calculator, log of 3.1 X 10-4 is equal to -3.51.
pH is equal to the negative log of hydrogen ion concentration. So, the negative of -3.51 gives us the pH of the solution to be equal to 3.51.
pOH is just the negative logarithm of hydroxyl ion concentration.
For example, what is the pOH of a solution that contains a hydrogen ion concentration [H+] of 9.5 X 10-10 M?
Calculating pOH using the hydrogen ion concentration [H+] is a bit more complicated. There are two ways of calculating pOH.
Water dissociates into hydrogen ion [H+] and hydroxyl ion [OH-].
The equilibrium constant Kw for this equation is expressed as
The first method for calculating pOH is a two-step process. The first step is to calculate the hydroxyl ion concentration using the auto-ionization expression for water.
And the second step is to use this value to calculate pOH.
The second method for calculating pOH is using the relationship between pH and pOH,
In the second method, the hydrogen ion concentration is used to calculate the pH. And pOH is calculated by simply subtracting the pH from 14 as shown in this expression.
For this problem, we will use the second method. In this problem, the concetration of hydrogen ions [H+] is 5 X 10-10 M.
Using a calculator, we get the negative logarithm of is 9.02. So, the pH of the solution is 9.02.
Using the relationship between pH and pOH, we get pOH is equal to 14 minus pH.
This will give us the value of pOH to be equal to 4.98.