Question

It is often possible to change a hydrate into an anhydrous compound by heating it to drive off the water (dehydration). A 45.91 gram sample of a hydrate of MnSO4 was heated thoroughly in a porcelain crucible, until its weight remained constant. After heating, 37.07 grams of the anhydrous compound remained. What is the formula of the hydrate

Answer 1

Answer: The formula of the hydrate is $MnSO_4.2H_2O$

Explanation:

Decomposition of hydrated manganese sulphate is given by:

$MnSO_4.xH_2O\rightarrow MnSO_4+xH_2O$

Molar mass of $MnSO_4$ = 151.0 g/mol

According to stoichiometry:

(151.0+18x) g of $MnSO_4.xH_2O$ decomposes to give 151.0 g of $MnSO_4$

Thus 45.91 g of $MnSO_4.xH_2O$ decomposes to give= $\frac{ 151.0}{151.0+18x}\times 45.91g$  of $MnSO_4$

Also we are given : 37.07 g of  $MnSO_4$ is produced

Thus we can equate the two equations:

$\frac{ 151.0}{151.0+18x}\times 45.91=37.07$

$x=2$  

Thus the formula of the hydrate is $MnSO_4.2H_2O$