Percent Composition Calculator
Each element's share of the mass — 90 g of every 250 in copper sulfate pentahydrate is water.
The formula
molar mass = Σ (atomic weight × atom count)
Weighted averages, not whole numbers
Chlorine's standard atomic weight is 35.45 because terrestrial chlorine is roughly three quarters chlorine-35 and one quarter chlorine-37.
No individual atom weighs 35.45. The figure describes a sample, and it is the right thing to use whenever you are weighing out bulk material rather than counting individual nuclei.
The mole exists to make this easy
Avogadro's constant was chosen so that a mole of an element weighs its atomic weight in grams.
That is the whole reason the number is 6.022 × 10²³ rather than something round. Since 2019 it is fixed by definition, so the relationship is now exact by construction rather than by measurement.
Read the dot as "plus"
CuSO₄·5H₂O is copper sulfate with five separate water molecules held in the crystal, not a single compound with subscripts multiplied through.
The water is genuinely there and genuinely weighs something — 90 grams of every 250 in copper sulfate pentahydrate is water, which is why the anhydrous salt is a different substance with a different mass.
Standard atomic weights are averages
The masses used here are weighted means over the isotopes found naturally on Earth, which is why chlorine is 35.45 rather than a whole number.
For work with a specific isotope, or with material from an unusual source, the isotopic mass is the right figure and it is not this one.