Question

If epsom salt \[MgS{O_4} \cdot X{H_2}O\] is heated to ${250^0}c$, all the water of hydration is lost. On heating a 1.687 g sample of the hydrate, 0.824 g of \[MgS{O_4}\] remains. How many molecules of water occur per formula unit of \[MgS{O_4}\]?
(Atomic mass: Mg = 24, S = 32, O =16)

Answer 1

Hint: Since, In \[MgS{O_4} \cdot \;X{H_2}O\], x molecule of water is present per formula unit of Epsom salt
And we can find the gram formula mass of Epsom salt. We will calculate the number of moles of
(\[MgS{O_4}\]) and number of moles of (\[{H_2}O\]).

Complete step by step answer:
In \[MgS{O_4} \cdot X{H_2}O\], x molecule of water is present per formula unit of Epsom salt
The gram formula mass of Epsom salt is \[ = \left\{ {924 + 32 + 16 \times 4) + 18x} \right\}g = \left( {120 + 18x} \right)2\]
We know,
Number of moles = Given weight / Molecular weight
Molecular weight of \[MgS{O_4} = 120g/mol\]
Molecular weight of \[{H_2}O = 18g/mol\]
Number of moles of \[(MgS{O_4}) = 0.824g/120g/mol = 0.0069mol\]
Number of moles of \[({H_2}O) = 1.687g/18g/mol-0.824g = 0.863\]
And number of moles of 0.863 g of \[\left( {{H_2}O} \right) = 0.863g/18g/mol = 0.048mol\]
\[x = 0.048mol/0.0069 = 7\]

Therefore, 7 molecules of water occur per formula unit of \[MgS{O_4}\].

Note: Magnesium sulphate (\[MgS{O_4}\]) is a magnesium salt having sulphate as the counter ion. It has a role as an anticonvulsant, a cardiovascular drug, a calcium channel blocker, an anaesthetic, a tocolytic agent, an antiarrhythmic drug, an analgesic and a fertilizer. It is a magnesium salt and a metal sulphate. A small colourless crystal used as an anticonvulsant, a cathartic, and an electrolyte replenisher in the treatment of pre-eclampsia and eclampsia. It causes direct inhibition of action potentials in myometrial muscle cells. Excitation and contraction are uncoupled, which decreases the frequency and force of contractions.