Hydrates arecompounds that incorporate water molecules into their fundamental solidstructure. In a hydrate (which usually has a specific crystalline form),a defined number of water molecules are associated with each formula unitof the primary material.
Gypsum is a hydratewith two water molecules present for every formula unit of CaSO4.The chemical formula for gypsum is CaSO4 2H2O andthe chemical name is calcium sulfate dihydrate. Note that the dot in theformula (or multiplication sign) indicates that the waters are there. Otherexamples of hydrates are: lithium perchlorate trihydrate - LiClO4 3H2O; magnesium carbonate pentahydrate - MgCO3 5H2O; and copper(II) sulfate pentahydrate - CuSO45 H2O.
The water in the hydrate(referred to as "water of hydration") can be removed by heating the hydrate.When all hydrating water is removed, the material is said to be anhydrousand is referred to as an anhydrate.
CuSO4 5 H2O(s) + HEAT ---> CuSO4 (s)+ 5 H2O (g)
hydrateanhydrate
Experimentally measuringthe percent water in a hydrate involves first heating a known mass of thehydrate to remove the waters of hydration and then measuring the mass ofthe anhydrate remaining. The difference between the two masses is the massof water lost. Dividing the mass of the water lost by the original massof hydrate used is equal to the fraction of water in the compound. Multiplyingthis fraction by 100 gives the percent water.
When a 1.000 g sampleof CuSO4 5 H2O(s) was heated so that the watersof hydration were driven off, the mass of the anhydrous salt remainingwas found to be 0.6390 g. What is the experimental value of the percentwater of hydration?
CuSO4 5 H2O(s) + HEAT ----> CuSO4 (s)+ 5 H2O (g)
1.000 g0.6390 g
1. Thedifference between the hydrate mass and anhydrate mass is the mass of waterlost.
1.000 g - 0.6390 g = 0.3610 g
2. Dividethe mass of the water lost by the mass of hydrate and multiply by 100.
(0.3610 g /1.000 g)(100) = 36.10%
The theoretical (actual)percent hydration (percent water) can be calculated from the formula ofthe hydrate by dividing the mass of water in one mole of the hydrate bythe molar mass of the hydrate and multiplying by 100.
What is the percentwater in copper(II) sulfate pentahydrate, CuSO4 5 H2O?
1. Calculatethe formula mass. When determining the formula mass for a hydrate, thewaters of hydration must be included.
(1 Cu)(63.55 g/mol) + (1 S)(32.07 g/mol) + (4 O)(16.00 g/mol) = 159.62g/mol
Formula mass = 159.62 g/mol + (5 H20)( 18.02 g H20/mol)= 249.72 g/mol
2. Divide themass of water in one mole of the hydrate by the molar mass of the hydrateand multiply this fraction by 100.
Percent hydration = (90.10 g /249.72 g)(100) = 36.08%
CHEMISTRY 103: PERCENT WATER INA HYDRATE
Put on your CHEMICALSPLASH-PROOF SAFETY GOGGLES!
Attach a secondsheet and clearly show all calculations.
PROCEDURE
1. Accuratelyweigh a clean, dry crucible. Record this mass to +0.01 g.
2. Transferapproximately 3 grams of barium chloride dihydrate, BaCl2 2H2O, into the weighed crucible and weigh the
crucible and its contents. Record this mass to +0.01 g.
3. Place thecrucible on a ring stand using a ring and clay triangle and heat gentlyfor 10 minutes. Then heat the sample
more strongly for 10 more minutes by bringing the flame of the bunsen burnerdirectly under the dish. The residue
should be almost pure white. Allow the crucible to cool, then weighit. Record this mass to +0.01 g.
4. Heat thecrucible for another 5 minutes, cool, then weigh. If all the waterhas been driven off, the two masses should
agree. Record this mass to +0.01 g.
5. Dispose ofthe barium chloride in the container provided.
DATA
1. Mass of emptycrucible _______________g
2. Mass of crucible& BaCl2 2 H2O _______________g
3. Mass of BaCl22 H2O _______________g (#2 - #1)
4. Mass of crucible& BaCl2 after first heating _______________g
5. Mass of BaCl2after first heating _______________g (#4 - #1)
6. Mass of crucible& BaCl2 after second heating _______________g
7. Mass of BaCl2after second heating _______________g (#6 - #1)
8. Mass of waterlost _______________g H2O
9. Percent hydration_______________% H2O
10. Theoreticalvalue _______________% H2O
11. Percent error_______________%
Atomic masses: H =1.01; O = 16.00; Cl = 35.45; Ba = 137.33
Reminders:
1. Bariumchloride is toxic. Use care when handling.
2. The usedbarium chloride should be put in the waste container provided.
It is very important that the evaporating dish cools to room temperaturebefore weighing. If it is not
cool, convection currents will be set up that will lower the mass.
3. The ringstand, ring, and crucible are hot. BE CAREFUL!!!!!
