Question

The crystalline salt Na2SO4.xH2O on heating loses 55.5% of its weight the formula of crystalline salt is??? a)Na2SO4.5H2O b)Na2SO4.7H2O c)Na2SO4.2H2O d)Na2SO4.10H2O e)Na2SO4.6H2O

Answer 1

Good question, I would opt for `(d)`...

Explanation:

We gots `Na_2SO_4*(OH_2)_n` where `n`, the number of so-called waters of crystallization are to be determined.....

We write the dehydration reaction as follows....

`Na_2SO_4*(OH_2)_n +Delta rarrNa_2SO_4 + nH_2O`

We assume we start with a `100*g` mass of sodium sulfate hydrate.....of which of course a `55.5*g` mass are the waters of solvation, and `45.5*g` are the `Na_2SO_4`.

`"Moles of sulfate,"` `=(45.5*g)/(142.04*g*mol^-1)=0.317*mol` `(i)`

`"Moles of waters,"` `=(54.5*g)/(18.01*g*mol^-1)=3.03*mol` `(ii)`

We divide `(ii)` by `(i)` to get the numbers of waters of crystallization, i.e. there are 10 moles of water per mole of sodium sulfate (or near enuff, I rounded up to 10 from 9.5).

And thus we got `Na_2SO_4*10H_2O`, the so-called `"decahydrate"`.

Anhydrous sodium sulfate is often used as a preliminary drying agent with which to treat wet solvents......