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Chemistry: Post your doubts here!

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2-
Mols of S2O3 = Mols of Cu.
Mols of S2O3= 0.02 = mol/(19.5/1000)
= 0.00039 mol

0.00039 mol in 50cm3
so [Cu2+] = 0.00039/(50/1000) = 0.0078 mol/dm3

mols of copper in 50cm3 = 0.00039
mols of copper in 100cm3 = 0.00078
mass of copper in 100cm3 = 0.00078*63.5 = 0.0495 (this value is mass / 100cm3 thus percentage)

So 0.0495% copper. If you don't get this method, let me know I'll tell you the alternative method.
 
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