Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryMARCH 20243 Marks
Question
Define acids and bases according to Bronsted$-$Lowry theory. Derive relationship between $pH$ and $pOH.$
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Answer
$\text{J. N.}$ Bronsted and $\text{T. M.}$ Lowry $(1923)$ proposed a more general theory known as the Bronsted$-$Lowry proton transfer theory. According to this theory acids and bases are defined as follows:
Acid: Acid is a substance that donates a proton $\left( H ^{\oplus}\right)$ to another substance.
Base: Base is a substance that accepts a proton $\left( H ^{\oplus}\right)$ from another substance.
We know that the ionic product of water is given as:
$K_W = [H_3O^+][OH^–]$
Now, $K_W = [H_3O^+][OH^–] = 1 \times 10^{–14}$
Taking $\log$ on both sides, we write
$log_{10} [H_3O^+] + log_{10} [OH^–]$
$= log_{10} (1 \times 10^{–14})$
$= – 14$
Multiply by $(– 1),$ we get,
$log_{10} [H_3O^+] + {log_{10} [OH^–]} = - 14$
$- log_{10} [H_3O^+] + {- log_{10} [OH^–]} = 14$
Now $pH = - log_{10} [H_3O^+]$ and $pOH = - log_{10} [OH^–]$
$\therefore pH + pOH = 14$
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