Answer: 25%
Explanation: To find the mass percentage of a chemical you need to do mass/total mass x 100 = %.
In this case you would do 5.0/20.00 because 5.0 is the mass of the hydrogen and 20.00 is the total mass. That equals .25 so the next step is to times that by 100.
.25 x 100 = 25, so we know that the percent of the hydrogen mass in compound 1 = 25%