A copper salt contains #25.5%# #Cu#, #12.9%# #S#, #25.6%# #O#, and #36.0%# #"water"#...what is the empirical formula of the hydrate?

1 Answer
May 31, 2016

Answer:

#CuSO_4*5H_2O#

Explanation:

We ASSUME that there are #100*g# of copper salt, and work out the molar quantities:

#Cu:# #(25.5*g)/(63.55*g*mol^-1)# #=# #0.401*mol# #Cu#.

#S:# #(12.9*g)/(32.06*g*mol^-1)# #=# #0.401*mol# #S#.

#O:# #(25.6*g)/(15.999*g*mol^-1)# #=# #1.60*mol# #O#

#H_2O:# #(36.0*g)/(18.01*g*mol^-1)# #=# #2.00*mol# #H_2O#

To get the empirical formula, we divide thru by the LOWEST molar quantity, i.e. #0.401*mol#, to give:

#CuSO_4*5H_2O#, #"copper sulfate pentahydrate"#

I have treated the water here as I would normally treat an element; I am certainly free to do so inasmuch as we could dry the salt and measure the actual water loss.

What is the colour of this salt?