Answer:
2.56 x 10^25
Step-by-step explanation:
1. convert grams to moles (divide by its molar mass)
molar mass of hydrogen = 1.01 g/mol
21.5g / 1.01 g/mol = 21.287 mol of H
2. convert moles to atoms (multiply by avogadro's number)
avogadro's number = 6.02 x 10^23
21.287 mol x (6.02 x 10^23) = 1.2815 x 10^25 atoms of H
3. Multiply by how many atoms in the compound
H2O has 2 hydrogen atoms, so multiply by 2
(1.2815 x 10^25) x 2 = 2.56297 x 10^25
4. Correct Significant Figures
There are 3 sig figs in 21.5g so its 3 sig figs
Correct Significant Figures = 2.56 x 10^25