Which method is used to collect $H_2$ gas in a gas jar in the laboratory?

Which of the following properties of $H_2$ (dihydrogen) gas is $NOT$ correct?

Balance the following chemical equations and identify the type of chemical reaction.
$(a)$ $Mg(s) + Cl_2(g) \to MgCl_2(s)$
$(b)$ $HgO(s) \xrightarrow{\text{Heat}} Hg(l) + O_2(g)$
$(c)$ $Na(s) + S(s) \xrightarrow{\text{Fuse}} Na_2S(s)$

Translate the following statements into chemical equations and then balance them.
$(a)$ Hydrogen gas combines with nitrogen to form ammonia.
$(b)$ Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
$(c)$ Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
$(d)$ Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
$(a)$ Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at $773 \ K$ to form ammonia gas.
$(b)$ Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.

Which gas is the lightest in weight?