The hardness of a water sample containing $10^{-3} \; M \; MgSO_{4}$ expressed as $CaCO_{3}$ equivalents (in $ppm$) is (molar mass of $MgSO_{4}$ is $120.37 \; g/mol$)

For the reaction, $2Al_{(s)} + 6HCl_{(aq)} \rightarrow 2Al^{3+}_{(aq)} + 6Cl^{-}_{(aq)} + 3H_{2(g)}$, which of the following statements is correct?

What is the mass of $33.6 \ dm^3$ of methane gas at $S.T.P.$?

$A$ gas mixture contains $25 \%$ He and $75 \% CH_4$ by volume at a given temperature and pressure. The percentage by mass of methane in the mixture is approximately . . . . . . (in $\%$)

Consider the reaction shown below where $6.1 \,g$ of benzoic acid is used to obtain $7.8 \,g$ of $m$-bromobenzoic acid. The percentage yield of the product is ....... .
(Round off to the Nearest integer) [Given : Atomic masses : $C = 12.0 \,u, H = 1.0 \,u, O = 16.0 \,u, Br = 80.0 \,u$]

$A$ $240 \ mL$ mixture of $CO_2$ and $CO$ is passed over red-hot coke. The $CO_2$ is completely reduced to $CO$. When the resulting gas is brought to the same temperature and pressure,the volume becomes $300 \ mL$. What is the volume of $CO$ in the original mixture in $mL$?