College Physics (4th Edition)

There are $2.65\times 10^{25}$ hydrogen atoms.
A carbon atom consists of 6 protons and 6 neutrons. A hydrogen atom consists of 1 proton. An oxygen atom consists of 8 protons and 8 neutrons. We can find the mass of one molecule of sucrose: $(12)(12~u)+(22)(1~u)+(11)(16~u) = 342~u$ The fraction of a sucrose molecule's mass that is hydrogen is $\frac{22~u}{342~u} = 0.0643$ We can find the mass of the hydrogen in 684.6 grams of sucrose: $m = (0.0643)(0.6846~kg) = 0.044~kg$ We can find the number of hydrogen atoms: $\frac{0.044~kg}{1~u} = \frac{0.044~kg}{1.66\times 10^{-27}~kg} = 2.65\times 10^{25}$ There are $2.65\times 10^{25}$ hydrogen atoms.