Answer: 9.8g
Explanation:
The calculation is based on the fact that all the water in the molecule will be removed.
i will, then, calculate the mass of water removed and then subtract it from the original 20.0 g of sample.
You can do that following these steps.
1) Calculate the number of moles of the hydrated magnsium sulfate, MgSO₄⋅7H₂O
number of moles = mass in grams / molar mass
The molar mass of MgSO₄⋅7H₂O is calcualted from the atomic mass of each atom times the number of atoms in the formula:
molar mass = 24.305 g/mol + 32.065 g/mol + 4×15.999g/mol + 7×2×1.008g/mol + 7×15.999 g/mol = 246.471 g/mol
⇒ moles of MgSO₄⋅7H₂O = 20.0g / 246.471 g/mol = 0.0811 moles
2) Calculate the number of moles of water, using the ratio from the chemical formula:
7 moles H₂O / 1 mol MgSO₄⋅7H₂O = x / 0.0811 mols MgSO₄⋅7H₂O
⇒ x = 0.0811 × 7 moles H₂O = 0.568 moles H₂O
3) Convert moles of H₂O to grams:
number of moles = mass in grams × molar mass = 0.568 moles × 18.015 g/mol = 10.2 g
4) Hence the mass remaining is 20.0g - 10.2g = 9.8g
