The mass percent of hydrogen in hydrogen peroxide, h2o2, is

The atomic weight of H is 1, O is 16.

There are two of each in a hydrogen peroxide molecule.

(1*2) + (16*2) = 2+32

2+32=34

The weight of the whole molecule is 34, and hydrogen makes up 2 of that.

(2/34) * 100=0.0588235294*100

0.0588235294*100=5.88235294

The percentage mass of hydrogen in a hydrogen peroxide molecule is 5.88%