Answer
There are $2.65\times 10^{25}$ hydrogen atoms.
Work Step by Step
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.
