Question
Sun September 11, 2011 By:

15.00 gm of Na2SO4.XH2O contains 2.36*1023 H2 molecules. Find the value of X in the water of crystallization.

Expert Reply
Wed September 14, 2011
Mass of Na2SO4 is 142.04 g/mol; Na2SO4.XH2O = 142 + x * 18 g/mol
 

1 mole Na2SO4.xH2O ----> x mole H2O

(18x+142) g Na2SO4.xH2O    --------->18x g H2O

15.00 g Na2SO4.xH2O -------->15.00 *18x/(18x+142) g H2O

 
Lets find out how many mols of compound is equivalent to 15 grams. Let it be y mols.
 
(1 mol Na2SO4. x H2O = 142 + 18x g;)  (15.00 g of Na2SO4. xH2O = 15/ (142 + 18x))
 
y = 15/ (142 + 18x) mol
y mols of compound has 2.36 * 1023 molecules of water.
1 mol of compound has 2.36/y * 1023 molecules of water.

According to the formula, 1 mol of compound has x mols of water.
So, x * 6.02 * 1023 molecules of water = 2.36/y * 1023
 
put the value of y and solve it.
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