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

1 Answer
anor277
Sep 6, 2017

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......

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