pH and pH Scale

pH of Solution:

pH:

pH is defined as the $- v e$ of logarithm (base $10$ ) of hydronium ion concentration.

$p H = - l o g [H^{+}]$

For simplicity, $H_{3} O^{+}$ is written as $H^{+}$
For pure water (neutral); $p H = 7$
For acidic solution; $p H < 7$
For basic solution; $p H > 7$
For very dilute solution, such as $1 0^{- 8} M$ $H^{+}$ , $p H = 8$ but in actual it is not true because we cannot neglect $1 0^{- 7} M$ $H^{+}$ from $H_{2} O$
The pH range is taken from $0 t o 14$ . However, pH may be negative for very strong acids.

pOH:

$pO H = - l o g [O H^{-}]$

and,

$pO H + p H = 14$

Note:

$p H = - l o g [H^{+}] = - l o g [M]$

For very dilute solution,

$p H = - l o g [H^{+}] = - l o g (c α) = - l o g (\frac{K a}{c} \times c)$ $= - l o g (K a c)$

It is the solution which resists the change in pH even after addition of small amount of acid or base.

pH of acidic buffer= $p K_{a} + l o g \frac{[ C o n c . o f S a lt ]}{C o n c . o f A c i d ]}$

$p K_{a} = - l o g K_{a}$

pH of basic buffer= $p K_{b} + l o g \frac{[ C o n c . o f S a lt ]}{C o n c . o f B a se}$

$p K_{b} = - l o g K_{b}$

A pH curve is a graph produced from a titration that shows how the pH of a solution changes when we add an acid or alkali to it.
The actual shape of curve depends upon the nature of alkali used for titration.

-End point ranges from $3 t o 11$ .

-Both phenolphthalein and methyl orange can be used as indicators.

-Take hydrochloric acid and sodium hydroxide as typical of a strong acid and a strong base.

$N a O H (A q) + H C L (A q) \to N a C L (A q) + H_{2} O (L)$

-End point ranges from $3 t o 8$ .

-Methyl orange can be used as indicators.

-Take hydrochloric acid as the strong acid and ammonia solution as the weak base.

$N H_{3} (A q) + H C L (A q) \to N H_{4} (A q)$

-End point ranges from $6 t o 11$ .

-Phenolphthalein can be used as indicators.

-Take ethanoic acid and sodium hydroxide as typical of a weak acid and a strong base.

$C H_{3} COO H (A q) + N a O H (A q) \to C H_{3} COON a (A q) + H_{2} O (L)$

-End point ranges from $6 t o 8$ .

-Such titration cannot be carried out due to the lack of suitable indicator.

-Take ethanoic acid and ammonia as typical of a weak acid and weak base.

$C H_{3} COO H (A q) + N H_{3} (A q) \to C H_{3} COON H_{4} (A q)$