How many moles of water molecules are in 6.81 g of magnesium sulfate heptahydrate (epsom salts

0.34 mol

Justification:

1) Hepta means 7, so heptahyrate means 7 parts of water. On mole of magnesium sulfate heptadydrate has seven moles of water.

2)  The formula of magnesium sulfate heptahydrate is:

Mg SO4 . 7 H2O

3) Calcualte the number of moles of the compound by dividing the mass in grams by the molar mass of formula.

molar mass:

Mg: 24.3 g/mol
S: 32.1 g/mol
O4: 4 * 16.0 g/mol = 64.0 g/mol
H2: 2 * 1.0 g/mol = 2.0 g/mol
O: 16 g/mol

Molar mass = 24.3 g/mol + 32.1 g/mol + 64.0 g/mol + 2.0 g/mol + 16.0 g/mol = 138.4 g/mol

number of moles = mass in grams / molar mass = 6.81 g / 138.4 g/mol = 0.049 mol

4) As stated, there are 7 moles of H2O per each mol of heptahydrate compoind, so the number of moles of water is 7 * 0.049 mol = 0.343 mol

0.34 mol