Here's how to determine the number of hydrogen atoms in 25 molecules of C4H4S2:

1. Identify the number of hydrogen atoms per molecule:

* The chemical formula C4H4S2 tells us that each molecule of this compound contains 4 hydrogen atoms.

2. Multiply the number of hydrogen atoms per molecule by the number of molecules:

* 4 hydrogen atoms/molecule * 25 molecules = 100 hydrogen atoms

Therefore, there are 100 hydrogen atoms in 25 molecules of C4H4S2.