By Jack Brubaker
Updated Aug 30, 2022

Copper sulfate pentahydrate (CuSO4·5H2O) is a hydrate, meaning the water molecules are chemically bonded to the anhydrous salt. A 100‑gram sample of the hydrate therefore contains both copper sulfate and water. To express how much copper sulfate is present, chemists use the mass‑percent (percent by mass) of the anhydrous component.

Step 1 – Determine the Formula Weight of CuSO4·5H2O

Using standard atomic weights (IUPAC), calculate the mass of each element in the formula and sum them:

• Cu = 63.55 amu × 1 = 63.55 amu
• S = 32.07 amu × 1 = 32.07 amu
• O = 16.00 amu × 9 = 144.00 amu
• H = 1.01 amu × 10 = 10.10 amu

Formula weight = 63.55 + 32.07 + 144.00 + 10.10 = 249.72 amu.

Step 2 – Determine the Formula Weight of Anhydrous CuSO4

Remove the water contribution:

• Cu = 63.55 amu × 1 = 63.55 amu
• S = 32.07 amu × 1 = 32.07 amu
• O = 16.00 amu × 4 = 64.00 amu

Formula weight = 63.55 + 32.07 + 64.00 = 159.62 amu.

Step 3 – Calculate the Mass‑Percent of CuSO4

Divide the anhydrous weight by the hydrate weight and multiply by 100 %:

159.62 / 249.72 × 100 ≈ 63.92 %

Thus, a 100‑gram sample of copper sulfate pentahydrate contains about 63.92 g of CuSO4 and 36.08 g of water.

Materials Needed

• Periodic table of the elements
• Scientific calculator

TL;DR

Use the atomic weights to compute the formula weights of CuSO4·5H2O and CuSO4, then divide and multiply by 100 to obtain the mass‑percent of anhydrous copper sulfate (≈63.9 %).