Hello!
Let's first look at glucose's molecular formula. This shows which atoms are in the molecule. Glucose's formula is C6H12O6. This means it has 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.
Now, you're looking for percent by mass. This means that we need to find the mass of the hydrogen atoms and the mass of the entire molecule. To do this, first find the atomic mass for each of the atoms in glucose.
Carbon's amu: 12.011
Hydrogen's amu: 1.008
Oxygen's amu: 15.999
Now, find the atomic mass for the entire molecule. To do this, multiply the amu's with the respective number of atoms, then add them all together.
Carbon: 6 atoms * 12.011 = 72.066
Hydrogen: 12 atoms * 1.008 = 12.096 (important for later)
Oxygen: 6 atoms * 15.999 = 95.994
Add them together: 72.066 + 12.096 + 95.994 = 180.156 amu for one glucose molecule.
We already know how much all the atoms of hydrogen weigh, as we did that when finding the weight of the glucose molecule. Restating it, it is 12.096.
Now, divide the mass of the hydrogen by that of glucose.
12.096 / 180.156 = 0.0671418104 = 6.714%
Therefore, your final answer is 6.714%.
Hope this helps!
