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Hydrate Calculations
Example 1: Calculate the percentage of water in Na2S2O3
. 5H2O
%H2O = x 100
= x 100
= 36.30 %
Mass of waterMass of hydrate
5(18.02 g/mol) 248.22 g/mol
Example 2: Calculate the mass of
water in 215 g of Na2S2O3. 5H2O
Solution 1: Use the percentage 36.30% (from the previous example).
In Na2S2O3. 5H2O, there is 36.30% of
water.In 215 g, there is x 215 g = 78.0 g of water
36.30 100
Solution 2: =
=
m(H2O) = 78.0 g
M(H2O)M (Total)
m(H2O)M(Total)
5(18.02 g/mol) 248.22 g/mol
m(H2O) 215 g
Example 2: A hydrate of barium chloride (BaCl2.x H2O) has a mass of 1.500g. When heated to drive off the water, the residue
(BaCl2) has a mass of 1.279 g
Step 1: Calculate the percentage of water in the hydrate.
BaCl2.x H2O (s) BaCl2 (s) + x H2O 1.500 g 1.279g 1.500g – 1.279g = 0.221 g
% H2O = x 100 = x 100 = 14.7%
Mass of waterMass of hydrate
0.221 g1.500 g
Step 2: Calculate the formula of the hydrate
Mole Calculation: Using n=m/M BaCl2 H2O
n = n= = 0.006140 mol = 0.0123 mol
1.279g208.3 g/mol
0.221 g18.02 g/mol
Ratio Calculation: Divide by the smallest number of moles to figure out the ratio between the atoms
BaCl2 H2O
= ==1 = 2Therefore the formula is BaCl2
. 2H2O
0.006140 mol0.006140 mol
0.0123 mol0.006140 mol