In the balanced equation of the following reaction, the ratio of $a / b$ is: $aCaCO_3 + bH_3PO_4 \rightarrow pCa_3(PO_4)_2 + qCO_2 + rH_2O$

The mass of water produced when $4 \ g$ of hydrogen reacts with $4 \ g$ of oxygen is .......... $g$.

During the process of fermentation,the number of moles of $CO_2$ liberated from one mole of glucose is

$A$ closed $10 \, L$ vessel contains $1 \, L$ water gas $(1:1 \, CO : H_2)$ and $9 \, L$ air ($20 \% \, O_2$ by volume) at $STP$. The contents of the vessel are ignited. The number of moles of $CO_2$ in the vessel is closest to $.... \, mol$.

When copper is reacted with a certain concentration of nitric acid,nitric oxide and nitrogen dioxide are produced in equal volumes according to the following reaction:
$xCu + yHNO_3 \to xCu(NO_3)_2 + NO + NO_2 + 3H_2O$
What are the stoichiometric coefficients $x$ and $y$?

If $0.50 \ mol$ of $CaCl_2$ is mixed with $0.20 \ mol$ of $Na_3PO_4$,the maximum number of moles of $Ca_3(PO_4)_2$ which can be formed,is