Step-by-step explanation:
HCN = Hydrogen cyanide
In standard conditions, the chemical equation of formation of hydrogen cyanide from, hydrogen gas, carbon and nitrogen gas.
The balanced chemical reaction is given as:

According to stoichiometry, 1 mole of hydrogen gas , 2 moles of carbon atom and 1 mole of nitrogen gas combined together to form 2 moles of hydrogen cyanide gas.