Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryIonic Equilibria4 Marks
Question
Define pH and pOH. Derive relationship between pH and pOH.
✓
Answer
(1) $pH$ : The negative logarithm, to the base 10 , of the molar concentration of hydrogen ions, $H ^{+}$is known as the $pH$ of a solution. $pH =-\log _{10}\left[ H ^{+}\right]$
(2) $pOH$ : The negative logarithm, to the base 10 , of the molar concentration of hydroxyl ions, $OH ^{-}$is known as the $pOH$ of a solution. $pOH =-\log _{10}\left[ OH ^{-}\right]$
Relationship between $pH$ and $pOH$ :
The ionic product of water, $Kw$ is given by,
$K _{ w }=\left[ H _3 O ^{+}\right] \times\left[ OH ^{-}\right]$
At $298 K K _{ w }=1 \times 10^{-14}$
$\therefore pK _{ w }=-\log _{10} K _{ w }=\log _{10} 1 \times 10^{-14}=14$
$\because\left[ H _3 O ^{+}\right] \times\left[ OH ^{-}\right]=1 \times 10^{-14}$
Taking logarithm to base 10 of both sides,
$\log _{10}\left[ H _3 O ^{+}\right]+\log _{10}\left[ OH ^{-}\right]=\log _{10} 1 \times 10^{-14}$
Multiplying both the sides by -1 ,
$-\log _{10}\left[ H _3 O ^{+}\right]-\log _{10}\left[ OH ^{-}\right]=-\log _{10} 1 \times 10^{-14}$
$\because pH =-\log _{10}\left[ H _3 O ^{+}\right] ; pOH =-\log _{10}\left[ OH ^{-}\right] ;$
$pK _{ w }=-\log _{10} K _{ w }$
$\therefore pH + pOH = pK _{ w }$
$OR \ \ pH + pOH =14$
Need a full question paper?
Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.