Which of the following is formed when calcium combines with oxygen?

In the reaction $2H_2 + O_2 \to 2H_2O$,if $6 \ g$ of $H_2$ combines with $64 \ g$ of $O_2$,find the mass of the excess reagent left in $g$.

Percentage loss in mass on heating a mixture of $Na_2CO_3$ and $CaCO_3$ containing equal mass of the two components will be ............. $\%$

$1 \ g$ of sodium hydroxide was treated with $25 \ mL$ of $0.75 \ M \ HCl$ solution. The mass of sodium hydroxide left unreacted is equal to:

If phosphoric acid is allowed to react with a sufficient quantity of $NaOH$,the product obtained is

Aluminium reacts with sulfuric acid to form aluminium sulfate and hydrogen. What is the volume of hydrogen gas in liters $(L)$ produced at $300 \ K$ and $1.0 \ atm$ pressure,when $5.4 \ g$ of aluminium and $50.0 \ mL$ of $5.0 \ M$ sulfuric acid are combined for the reaction?
(Use molar mass of aluminium as $27.0 \ g \ mol^{-1}, R = 0.082 \ atm \ L \ mol^{-1} \ K^{-1}$)