The reactants of the reaction of single replacement that produces ZnCl₂ and H₂ is Zn + 2HCl
Explanation
In a single replacement reaction is a chemical reaction whereby an element react with compound and take the place of another element in that compound.
zn displaces H from HCl to form ZnCl₂ and H₂ according to below equation.
Zn +2HCl → ZnCl₂ + H₂
At STP
1L O₂ → 2L CO₂
xL O₂ → 5.0L CO₂
x=2.5 L
According to Charles' Law the volume of an ideal gas is directly proportional to its absolute temperature in Kelvin keeping the pressure constant.
V∝ T, P is constant
where V, T and P are volume, temperature and pressure
= 
where V₁, T₁, V₂ and T₂ are initial volume, initial temperature, final volume and final temperature.