Name: pH Scale
Uploaded: 2019-07-19
Duration: 9 min 45 s
Description: This video discusses the pH of acids, bases, and salts and also the limitations of using a pH scale for identification. The natural indicators do the verific...

Question

Explain the terms ‘Ionic product of water’ and ‘

pH

value’. How does the former change with temperature? What is

pH

scale?

Accepted Answer

The ionic product of water $(K_{w})$ is defined as the product of the molar concentration of $H^{+}$ ions and ${OH}^{-}$ ions. As $H^{+}$ ions in water (or in an aqueous solution) exist as $H_{3} O^{+}$ ions, therefore, ionic product may also be defined as the product of molar concentrations of $H_{3} O^{+}$ and ${OH}^{-}$ ions.
Water is a weak electrolyte, i.e., it is ionized to a very small extent as

$H_{2} O ⇌ H^{+} + {OH}^{-}$ or $H_{2} O + H_{2} O \to H_{3} O^{+} + {OH}^{-}$

This ionization is called self-ionization of water.
Applying the law of chemical equilibrium to the above equilibrium, we get
$K = \frac{[H^{+}] [{OH}^{-}]}{[H_{2} O]}$ ............. $(1)$
Where, $K$ is the dissociation constant of water.
The concentration of unionized water molecules, i.e., $[H_{2} O]$ remains almost constant $($ being equal to $1000 / 18 = 55.55$ moles per liter, because $1 L$ of water $pH$ $pH$ and molar mass of $H_{2} O = 18 {gmol}^{- 1})$ i.e., $[H_{2} O] =$ constant
The equation $(1)$ may be rewritten as
$K = \frac{K_{w}}{55.55}$
It may be noted that ionic product of water is constant only at constant temperature. Its value at $25 ° C$ $(298 K)$ is found to be $1.008 \times 10^{- 14}$ . However, for simplicity, the value of $K_{w}$ at $298 K$ is usually taken as $K_{w} = 1.00 \times 10^{- 14}$ .

Effect of temperature on $K_{w}$
The ionic product of water $(K_{w})$ increases with increase of temperature. This is because, that with increase of temperature, the degree of ionization of water increases. Thus, the concentration of $H^{+}$ (or $H_{3} O^{+}$ ) ions and ${OH}^{-}$ ions increases and hence the ionic product also increases.

$p H$ may be defined as negative logarithm of hydronium(or hydrogen) ion concentration.
$p H = - l o g [H_{3} O^{+}]$
or $p H = l o g \frac{1}{[H_{3} O^{+}]}$ .

Theoretically, a solution may have any $H_{3} O^{+}$ ion concentration and hence may have any $pH$ value. However, practically, the solutions having $H_{3} O^{+}$ ion or ${OH}^{-}$ ion concentration more than $10^{0} (= 1 M)$ and hence, ${OH}^{-}$ ion or $H_{3} O^{+}$ ion concentration less than $10^{- 14} M$ are not common. This is because such solutions are not easy to obtain and moreover as the concentration increases, the degree of dissociation of the acid or the base decreases enormously. Hence, the $pH$ range is taken as $0$ to $14$ for most of the practical purposes. The acidity or the alkalinity of a solution may be represented on the $pH$ scale as follows:

Explain the terms ‘Ionic product of water’ and ‘ $pH$ value’. How does the former change with temperature? What is $pH$ scale?

Important Questions on Ionic Equilibrium in Solution

Learn and Prepare for any exam you want !

Explain the terms ‘Ionic product of water’ and ‘pH value’. How does the former change with temperature? What is pH scale?

Important Questions on Ionic Equilibrium in Solution

Learn and Prepare for any exam you want !

Explain the terms ‘Ionic product of water’ and ‘ $pH$ value’. How does the former change with temperature? What is $pH$ scale?