The reaction of sulphur in alkaline medium is the below:
$\mathrm{S}_{8(\mathrm{~s})}+\mathrm{a} \mathrm{OH}_{(\mathrm{aq})}^{-} \rightarrow \mathrm{b} \mathrm{S}^{2-}_{(\mathrm{aq})}+\mathrm{c} \mathrm{S}_{2} \mathrm{O}_{3}{ }^{2-}{ }_{(\mathrm{aq})}+\mathrm{d} \mathrm{H}_{2} \mathrm{O}_{(\ell)}$
The values of 'a' is (Integer answer)
for balancing in basic medium add equal number of $\mathrm{OH}^{\ominus}$ that of $\mathrm{H}^{+}$
$2 \mathrm{~S}_{8}+12 \mathrm{H}_{2} \mathrm{O}+24 \mathrm{OH}^{\ominus} \longrightarrow 8 \mathrm{~S}^{2-}+4 \mathrm{~S}_{2} \mathrm{O}_{8}^{2-}$
$+24 \mathrm{H}_{2} \mathrm{O}$
$2 \mathrm{~S}_{8}+24 \mathrm{OH}^{\ominus} \rightarrow 8 \mathrm{~S}^{2-}+4 \mathrm{~S}_{2} \mathrm{O}_{8}^{2-}+12 \mathrm{H}_{2} \mathrm{O}$
$\mathrm{S}_{8}+12 \mathrm{OH}^{\ominus} \rightarrow 4 \mathrm{~S}^{2-}+2 \mathrm{~S}_{2} \mathrm{O}_{8}^{2-}+6 \mathrm{H}_{2} \mathrm{O}$
$a=12$
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