In finding the empirical formula of a hydrate, a crucible sample was heated and
ID: 1018768 • Letter: I
Question
In finding the empirical formula of a hydrate, a crucible sample was heated and the final mass was measured. How would you determine in the labratory when to stop heating the cruciable, to say that the dehydration (removal of water) is complete?
The follwing data was collected while heating a hydrate, MX*H2O, in a crucible. Determine the emperical formula of the hydrate given that the salt (MX) molar mass is 131.66 g.mol.
(Empty Crucible: 85.000g
Crucible + hydrate mass before heating: 91.500 g
Crucible +pure salt mass after complete heating:89.200g)
Thank you so much for helping me and can you make sure to show all your calculations and describe so I can no why the answer is what it is!:)
Explanation / Answer
In order to get the accurate mass of the crucible and its contents, we need to reiterate the heating process until the value for mass on weighing the crucible comes out to be constant, i.e let us say the mass after heating it for the first time comes out to be 'x' grams, note down the reading and repeat the process, if you again get the same 'x' value for mass, then take the mass to be 'x' grams only. But if the value comes out to be a little different, repeat the process until you get a concordant reading.
Given that the mass of empty crucible is 85.000g
Mass of hydrated salt would be = 91.500g - 85.000g = 6.500g
And mass of pure salt is = 89.200g - 85.000= 4.200g
Now, the total mass of water content in the salt = 6.500g - 4.200g = 2.300g
Moles of water present in the hydrated salt= 0.1277 moles
Total mass of pure salt in the crucible= 4.200g
Moles of pure salt present in the crucible= 0.0319 moles
Simplifying the above digits,
for 1 mole of pure salt, there are 4 moles of water molecules attached to it.
Thus, the Empirical formula of the hydrate is : MX.4H2O