What is the weight of $H_2SO_4$ (in $g$) required to completely dissolve $3 \ g$ of magnesium carbonate?

$4.4 \ g$ of a gas at $STP$ occupies a volume of $2.24 \ L$. The gas can be:

Air contains $23\%$ oxygen and $77\%$ nitrogen by weight. What is the percentage by volume of $O_2$?

To a $25 \, mL$ of $H_2O_2$ solution,an excess of acidified solution of $KI$ was mixed. The liberated $I_2$ required $20 \, mL$ of $0.3 \, M$ hypo $(Na_2S_2O_3)$ solution for neutralization. The volume strength of $H_2O_2$ is......$mL$.

In the reaction shown,$5 \ g$ of toluene is converted into benzaldehyde with $92 \%$ yield. The amount of benzaldehyde produced is $.... \times 10^{-2} \ g$. (Nearest integer)

The stoichiometric reaction of $516 \ g$ of dimethyldichlorosilane with water results in a tetrameric cyclic product $X$ in $75 \%$ yield. The weight (in $g$) of $X$ obtained is. . . . . . . [Use,molar mass $(g \ mol^{-1})$: $H=1, C=12, O=16, Si=28, Cl=35.5$]