pH and pOH calculations — lesson. Science State Board, Class 10.

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How to calculate the $pH$ of a solution?

The

$pH$ is the negative logarithm of the hydrogen ion concentration.

i.e,

$pH= -log_{10}[H^+]$

Example:

Calculate the

$pH$ of

$0.01$

$M$

$HNO_3$ .

Solution:

$[H^+] = 0.01$

$pH = -log_{10}$

$[H^+]$

$pH = -log_{10}$

$[0.01]$

$pH = -log_{10}$

$[1\times10^{-2}]$

$pH = -(log_{10}·1 - 2\, log_{10}10)$

$pH = 0 + 2\times\,log_{10}\,10$

$pH = 0 + 2\times1 = 2$

$pH = 2$

pOH

The

$pH$ is related to the

$pOH$ of an aqueous solution.

The

$pOH$ is the negative logarithm of the hydroxyl ion concentration.

$pOH= -log_{10}[OH^-]$

Example:

The hydroxyl ion concentration of a solution is

$1\times10^{-9}$

$M$ . What is the

$pOH$ of the

solution?

Solution:

$pOH = -log_{10}[OH^-]$

$pOH = -log_{10}[1\times10^{-9}]$

$pOH = -(log_{10}\times1.0 + log_{10}\times10^{-9})$

$pOH = -(0 - 9\,log_{10}\,10)$

$pOH = -(0 - 9)$

$pOH = 9$

Relationship between $pH$ and $pOH$ :

$25°C$ , the

$pH$ and

$pOH$ of a water solution are related by the following equation:

$pH + pOH = 14$

It is possible to calculate the other value if the

$pH$ or

$pOH$ of a solution is known.

Example:

$pOH$ of a solution is

$11.76$ . What is the

$pH$ of this solution?

$pH = 14 - pOH$

$pH = 14 – 11.76 = 2.24$

$pH$ indicates acidity or hydrogen ion concentration, whereas

$pOH$ indicates alkalinity or hydroxide ion concentration.

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8. pH and pOH calculations

Theory: