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

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.