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Q. What is the $\left[ OH ^{-}\right]$concentration of a $0.04 M$ solution of $CH _{3} COONa$ ? [$K _{ a }$ of $CH _{3} COONa =2\times 10^{-5}, \log 2=0.30, \log 5=0.69,10^{0.7} \cong 5$]

NTA AbhyasNTA Abhyas 2020Equilibrium

Solution:

$\mathrm{pK}_{\mathrm{a}}=-\log \mathrm{K}_a=-\log \left(2 \times 10^{-5}\right)$
$=5-\log 2=5-0.3=4.7$
$\mathrm{pH}=\frac{1}{2}\left(\mathrm{pK}_{\mathrm{w}}+\mathrm{pK}_{\mathrm{a}}+\log \mathrm{C}\right)$
$p H=\frac{1}{2}[14+4.7+\log (0.04)]$
$p H=8.63$
$pOH =14-8.63 \cong 5.3$
${\left[ OH ^{-}\right]=5 \times 10^{-6} }$