What is the weight (in $g$) of $Na_2CO_3$ (molar mass $= 106$) present in $250 \ mL$ of its $0.2 \ M$ solution?

$A$ $10 \ g$ effervescent tablet containing sodium bicarbonate $(NaHCO_3)$ and oxalic acid $(H_2C_2O_4)$ releases $0.25 \ L$ of $CO_2$ at $T = 298.15 \ K$ and $p = 1 \ bar$. If the molar volume of $CO_2$ is $25.0 \ L \ mol^{-1}$ under these conditions,what is the percentage by mass of sodium bicarbonate in each tablet? [Molar mass of $NaHCO_3 = 84 \ g \ mol^{-1}$,Molar mass of $H_2C_2O_4 = 90 \ g \ mol^{-1}$]

$15 \ mL$ of $0.2 \ N$ alkali is required to complete neutralization of $30 \ mL$ acid solution. The concentration of the acid solution is ..... $N$.

$X \ g$ of nitrobenzene on nitration gave $4.2 \ g$ of $m-$dinitrobenzene. $X = ............ \ g$. $(nearest \ integer)$ [Given: molar mass (in $g \ mol^{-1}$) $C: 12, H: 1, O: 16, N: 14$]

$20 \ g$ of hydrogen gas $(H_2)$ is present in a $5 \ L$ vessel. The molar concentration of hydrogen is: (in $M$)

What is the volume (in litres) of oxygen required at $STP$ to completely convert $1.5 \ mol$ of sulphur into sulphur dioxide?