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pH is a measure of how acidic or basic a solution is. It uses a scale that typically ranges from 0 to 14, with 7 being neutral. The formula to calculate pH is: \[ pH = -\text{log}[\text{H}_3\text{O}^+] \]
The pH value is derived from the concentration of hydronium ions (\[ \text{H}_3\text{O}^+ \] ) in the solution.
A pH less than 7 indicates an acidic solution, a pH of 7 indicates a neutral solution, and a pH greater than 7 indicates a basic solution.
This scale is logarithmic, which means that each whole number change on the pH scale corresponds to a tenfold change in H₃O⁺ concentration.

For example, if the concentration of hydronium ions in a solution is \[ 3.4 \times 10^{-2} \text{ M} \] , the pH can be calculated as follows: \[ pH = -\text{log}(3.4 \times 10^{-2}) = 1.47 \] Consequently, the solution is acidic because the pH is less than 7.

pOH is similar to pH but measures the concentration of hydroxide ions (\[ \text{OH}^- \] ). The pOH scale also ranges from 0 to 14. The relationship between pH and pOH can be expressed as: \[ pH + pOH = 14 \] .
The formula for calculating pOH is: \[ pOH = -\text{log}[\text{OH}^-] \]
Just like pH, a lower pOH indicates a basic (alkaline) solution, while a higher pOH indicates an acidic solution.
Note that knowing either the pH or pOH of a solution allows you to calculate the other using the above relationship.
For example, if the concentration of \[ \text{OH}^- \] ions is provided as \[ 6.3 \times 10^{-4} \text{ M} \], we calculate its pOH: \[pOH = -\text{log}(6.3 \times 10^{-4}) = 3.20 \].
Then, using the relationship between pH and pOH:\[pH = 14 - pOH = 14 - 3.20 = 10.80 \].
Since the pH is greater than 7, the solution is basic.

Hydronium ions (\[ \text{H}_3\text{O}^+ \] ) are formed when an acid dissolves in water and releases hydrogen ions (\[ \text{H}^+ \] ) which then bond with water molecules (\[ \text{H}_2\text{O} \]).
These ions are central to the concept of pH.
The concentration of hydronium ions in a solution determines its acidity.
For instance, an increased concentration of \[ \text{H}_3\text{O}^+ \] ions means a lower pH and greater acidity.
Conversely, a decreased concentration of \[ \text{H}_3\text{O}^+ \] ions means a higher pH and less acidity.
When the concentration of hydronium ions in a solution needs to be determined, such as in the case of \[ \text{H}_3\text{O}^+ = 1.9 \times 10^{-7} \text{ M} \], the corresponding pH can be calculated as:\[ pH = -\text{log}(1.9 \times 10^{-7}) = 6.72 \]. This solution is slightly acidic because the pH is slightly less than 7.

Hydroxide ions (\[ \text{OH}^- \] ) are produced when basic compounds dissolve in water.
They play a key role in determining the pOH and, consequently, the pH of a solution.
Higher concentrations of hydroxide ions indicate a basic solution with a lower pOH and higher pH.
One primary formula used involving hydroxide ions is:\[ pOH = -\text{log}[\text{OH}^-] \].
For instance, if the concentration of \[ \text{OH}^- \] ions is \[ 2.5 \times 10^{-10} \text{ M} \], the pOH can be calculated as:\[pOH = -\text{log}(2.5 \times 10^{-10}) = 9.60 \].
Using the formula to find pH from pOH:\[ pH = 14 - pOH = 14 - 9.60 = 4.40 \].
Because the pH is less than 7, the solution is acidic even though we started with hydroxide ions.

Acidic and basic solutions are characterized by their pH values.
If a solution has a pH less than 7, it is acidic.

• An example is a solution with \[ \text{H}_3\text{O}^+ \] concentration of \[ 3.4 \times 10^{-2} \text{ M} \], where the calculated pH is \[ 1.47 \]. This shows strong acidity.
• Another example is a solution with \[ \text{H}_3\text{O}^+ \] concentration of \[ 1.9 \times 10^{-7} \text{ M} \], having a calculated pH of 6.72, depicting slight acidity close to neutrality.

On the other hand, for basic solutions, the pH is greater than 7.
An example of a basic solution is when \[ \text{OH}^- \] concentration is \[ 6.3 \times 10^{-4} \text{ M} \] and the calculated pH is \[ 10.80 \].
This indicates a basic or alkaline solution.
Always remember, the characteristic of the solution changes with the pH value, and simple calculations using the pH and pOH concepts help in accurate determination.