The amount of anhydrous $Na_2CO_3$ present in $250 \ mL$ of $0.25 \ M$ solution is ........ $g$.

Change in volume when $10 \, mL$ of phosphine $(PH_3)$ is decomposed according to the reaction: $2PH_{3(g)} \rightarrow 2P_{(s)} + 3H_{2(g)}$.

$A$ spherical ball of radius $7 \ cm$ contains $56 \ w\%$ iron. Find out the number of moles of iron present approximately in the ball (in $.1$)? $(d = 1.4 \ g \ cm^{-3}; \text{atomic mass} = 56 \ g \ mol^{-1})$

What is the volume of oxygen required for complete combustion of $0.25 \ mole$ of methane at $\text{S.T.P.}$ (in $dm^3$)?

How many liters of $H_2$ gas will be evolved at $STP$ by the reaction of zinc metal with $500 \, mL$ of $0.5 \, N \, HCl$?

The molarity of a $1\,N\, H_3PO_3$ solution will be ......... (in $, M$)