What is obtained when one mole of calcium phosphide is reacted with excess water?

What is the percentage atom economy when the formula weight of the desired product is $65 \text{ u}$ and the sum of the formula weights of all reactants is $130 \text{ u}$ (in $\%$)?

Calcium carbonate reacts with aqueous $HCl$ to give $CaCl_{2}$ and $CO_{2}$ according to the reaction,
$CaCO_{3(s)} + 2HCl_{(aq)} \rightarrow CaCl_{2(aq)} + CO_{2(g)} + H_{2}O_{(l)}$
What mass of $CaCO_{3}$ is required to react completely with $25 \ mL$ of $0.75 \ M$ $HCl$ (in $g$)?

Molarity of a given orthophosphoric acid solution is $3 \ M$. Its normality is (in $N$)

$A$ mixture of $H_2$ and $O_2$ at $1 \, atm$ pressure contains $20 \%$ by weight of $H_2$. Find out the partial pressure of $H_2$ in $atm$.

The heat of combustion of ethene $(C_2H_4)$ is $1411 \, kJ$. If a certain amount of ethene is burned to produce $6226 \, kJ$ of heat,what is the volume of $O_2$ (at $S.T.P.$) consumed in the process?