$A$ sample of $MgCO_3$ is dissolved in dil. $HCl$ and the solution is neutralized with ammonia and buffered with $NH_4Cl / NH_4OH$. Disodium hydrogen phosphate reagent is added to the resulting solution. $A$ white precipitate is formed. What is the formula of the precipitate?

When $100 \ mL$ of $1.0 \ M \ HCl$ was mixed with $100 \ mL$ of $1.0 \ M \ NaOH$ in an insulated beaker at constant pressure,a temperature increase of $5.7^{\circ} C$ was measured for the beaker and its contents (Expt. $1$). Because the enthalpy of neutralization of a strong acid with a strong base is a constant $\left(-57.0 \ kJ \ mol ^{-1}\right)$,this experiment could be used to measure the calorimeter constant. In a second experiment (Expt. $2$),$100 \ mL$ of $2.0 \ M$ acetic acid $\left(K_a=2.0 \times 10^{-5}\right)$ was mixed with $100 \ mL$ of $1.0 \ M \ NaOH$ (under identical conditions to Expt. $1$) where a temperature rise of $5.6^{\circ} C$ was measured.
(Consider heat capacity of all solutions as $4.2 \ J \ g ^{-1} K ^{-1}$ and density of all solutions as $1.0 \ g \ mL ^{-1}$)
$1.$ Enthalpy of dissociation (in $kJ \ mol ^{-1}$) of acetic acid obtained from the Expt. $2$ is
$(A) \ 1.0 \ (B) \ 10.0 \ (C) \ 24.5 \ (D) \ 51.4$
$2.$ The $pH$ of the solution after Expt. $2$ is
$(A) \ 2.8 \ (B) \ 4.7 \ (C) \ 5.0 \ (D) \ 7.0$
Give the answer for question $1$ and $2.$

$A$ soft drink was bottled with a partial pressure of $CO_2$ of $3 \ bar$ over the liquid at room temperature. The partial pressure of $CO_2$ over the solution approaches a value of $30 \ bar$ when $44 \ g$ of $CO_2$ is dissolved in $1 \ kg$ of water at room temperature. The approximate $pH$ of the soft drink is............... $\times 10^{-1}$ (First dissociation constant of $H_2CO_3 = 4.0 \times 10^{-7}$; $\log 2 = 0.3$; density of the soft drink $= 1 \ g \ mL^{-1}$)

Equal volumes of aqueous solution of $0.1 \ M \ HCl$ and $0.2 \ M \ H_2SO_4$ are mixed. The concentration of $H^{+}$ ions in the resulting solution is (in $M$)

Assign $A, B, C, D$ from the given type of reaction.
$BaCO_3 \downarrow + CO_2 + H_2O \longrightarrow Ba(HCO_3)_2$

$HgCl_2$ and $I_2$ both when dissolved in water containing $I^{-}$ ions,the pair of species formed is