Calculate the volume of hydrogen chloride that is formed in the explosion of a mixture of chlorine and 10 liters of hydrogen.

We write the equation:
H2 + Cl2 = 2HCl
In the reaction equation, we can notice that the volumes of hydrogen and chlorine are equal. If the mixture is equimolar, then in a mixture of 10 liters of hydrogen, it will react with 10 liters of chlorine.
Also, according to the reaction, we can notice that the volume of the formed hydrogen chloride is 2 times greater than the volume of any of the reacting gases.
From this he concludes that the volume of HCl = 2 * 10 = 20 (l)