Mass of magnesium required to produce 220 mL of hydrogen gas at STP on reaction with excess of dil. HCl is

Given: Molar mass of Mg is $24 \mathrm{~g} \mathrm{~mol}^{-1}$.

In Dumas' method for estimation of nitrogen 0.4 g of an organic compound gave 60 mL of nitrogen collected at 300 K temperature and 715 mm Hg pressure. The percentage composition of nitrogen in the compound is (Given : Aqueous tension at $300 \mathrm{~K}=15 \mathrm{~mm} \mathrm{Hg}$ )

Among $10^{-9} \mathrm{~g}$ (each) of the following elements, which one will have the highest number of atoms?

Element: $\mathrm{Pb}, \mathrm{Po}, \mathrm{Pr}$ and Pt

$$\mathrm{CaCO}_3(\mathrm{~s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{CaCl}_2(\mathrm{aq})+\mathrm{CO}_2(\mathrm{~g})+\mathrm{H}_2 \mathrm{O}(\mathrm{l})$$

Consider the above reaction, what mass of $\mathrm{CaCl}_2$ will be formed if 250 mL of 0.76 M HCl reacts with 1000 g of $\mathrm{CaCO}_3$ ?

(Given : Molar mass of $\mathrm{Ca}, \mathrm{C}, \mathrm{O}, \mathrm{H}$ and Cl are $40,12,16,1$ and $35.5 \mathrm{~g} \mathrm{~mol}^{-1}$, respectively)