FeSO₄*7H₂O(s) = FeSO₄(s) + 7H₂O(g)
M(FeSO₄*7H₂O)=278.0 g/mol
M(FeSO₄)=151.9 g/mol
m(FeSO₄*7H₂O)/M(FeSO₄*7H₂O)=m(FeSO₄)/M(FeSO₄)
m(FeSO₄)=M(FeSO₄)m(FeSO₄*7H₂O)/M(FeSO₄*7H₂O)
m(FeSO₄)=151.9*100.0/278.0=54.6 g
m(FeSO₄)=54.6 g
The oil slick thick = 1.256 x 10⁻⁴ cm
<h3>Further explanation</h3>
Volume is a derivative quantity derived from the length of the principal
The unit of volume can be expressed in liters or milliliters or cubic meters
The conversion is
1 cc = 1 cm3
1 dm = 1 Liter
1 L = 1.06 quart
<em>so for 1 quart = 0.943 L</em>

Volume of oil dumped = volume of swimming pool

The concentration of sodium and sulphate ions are [
] = 0.4 M, [
] = 0.2 M
Explanation:
The molar concentration is defined as the number of moles of a molecule or an ion in 1 liter of a solution.
In the given solution, the concentration of the salt sodium sulphate is 0.2M. So, 0.2 moles of sodium sulphate is present in 1 liter of solution.
Assuming 100% dissociation,
1 molecule of sodium sulphate gives 2 ions of sodium and 1 ion of sulphate.
So 0.2 moles of sodium sulphate will give 0.4 moles of sodium ions and 0.2 moles of sulphate ions.