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