KVPY SA Exam 19-Nov-2017 Question Paper

Question : 74 of 80

Marks: +1, -0

A compound has the following composition by weight ; Na = 18.60 %, S = 25.80 %, H = 4.02 % and O = 51.58 % Assuming that all the hydrogen atoms in the compound are part of water of crystallization, the correct molecular formula of the compound is

$Na_2S_2O_3 .3H_2O$
$Na_2SO_4 .5H_2O$
$Na_2SO_4 .10H_2O$
$Na_2S_2O_3 .5H_2O$

Validate

Solution:

Elements	%	moles	Simplest ratio
Na	18.6	$18.6/23=0.8$	1 × 2
S	25.8	$25.8/32=0.8$	1 × 2
O	51.58	$51.58/16=3.22$	4 × 2
H	4.02	$4.02/1=4.02$	5 × 2

⇒ Formula is

Na_2S_2O_3 . 5H_2O

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