Which shows weak ionisation in water

$(I)\,\,B{r^ - }{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} $ $(II)\,\,\begin{array}{*{20}{c}}
{{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} O{\mkern 1mu} } \\
{{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} ||} \\
{{H_3}C - C - {O^ - }}
\end{array}$ $(III)\,\,C{H_3}C{H_2}{O^ - }$ $(IV)\,\,Ph - {O^ - }$

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Substance $A_2B(g)$ can undergoes decomposition to form two set of products If the molar ratio of $A_2(g)$ to $A(g)$ is $5 : 3$ in a set of product gases, then the energy involved in the decomposition of $1\, mole$ of $A_2B(g)$ is

${A_2}B(g) \to {A_2}(g) + B(g);\,\,\,\Delta {H^o} = 40\,kJ/mol$

${A_2}B(g) \to A(g) + AB(g);\,\,\,\Delta {H^o} = 50\,kJ/mol$

.....$ kJ/mol$

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One mol of methanol when burnt in excess of $O_2$ gives out $723 \,kJ\, mol^{-1}$ Heat. If $1\, mol$ of $O_2$ is used, what will be the amount of heat evolved......$kJ$

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$K _{ a_1,}, K _{ a_2 }$ and $K _{ a_3}$ are the respective ionization constants for the following reactions $(a), (b),$ and $(c)$.

$(a)$ $H _{2} C _{2} O _{4} \rightleftharpoons H ^{+}+ HC _{2} O _{4}^{-}$

$(b)$ $HC _{2} O _{4}^{-} \rightleftharpoons H ^{+}+ HC _{2} O _{4}^{2-}$

$(c)$ $H _{2} C _{2} O _{4} \rightleftharpoons 2 H ^{+}+ C _{2} O _{4}^{2-}$