To determine how many atoms of chlorine (Cl) reacted with the hydrogen (H) to produce hydrochloric acid (HCl), we can analyze the reaction:
The chemical equation for the reaction is: \[ \text{H}_2 + \text{Cl}_2 \rightarrow 2 \text{HCl} \]
From the problem, we know that 20 molecules of hydrochloric acid (HCl) are produced. Each molecule of HCl consists of one hydrogen atom and one chlorine atom.
Since 20 molecules of HCl are produced, this means:
- Hydrogen atoms used: 20 molecules of HCl will require 20 hydrogen atoms (because there is 1 hydrogen atom in each HCl molecule).
Now, for the chlorine atoms, since each molecule of HCl contains one chlorine atom, to create 20 molecules of HCl, we need:
- Chlorine atoms needed: 20 chlorine atoms (1 for each of the 20 HCl molecules).
So, according to the law of conservation of matter, since 20 hydrogen atoms react with 20 chlorine atoms, the number of chlorine atoms that reacted with the hydrogen is 20.
Therefore, the answer is C. 20.