Mole ConceptHard
Question
Calculate the milimoles of SeO32- in solution on the basis of following data :
70ml of
solution of KBrO3 was back titated with 12.5 mL of 
solution of 2 NaAsO2
The reaction are given below.
I. SeO32- + BrO3- + H+ → SeO42-Br2 + H2O
II. BrO3- + AsO2- + H2O → Br- + AsO43- + H+
70ml of
solution of KBrO3 was back titated with 12.5 mL of solution of 2 NaAsO2The reaction are given below.
I. SeO32- + BrO3- + H+ → SeO42-Br2 + H2O
II. BrO3- + AsO2- + H2O → Br- + AsO43- + H+
Options
A.1.6 × 10-3
B.1.25
C.2.5 × 10-3
D.None of these
Solution
(I) 
(II)
In reaction (II)
gm.eq. of BrO3- = gm.eq.of AsO2-
nBrO3- × 6 = nAsO2- × 2
=
× 2 = 10-3
nBrO3- =
In reaction (I)
moles of BrO3- consumed
= 10-3
gm eq. of SeO32- = gm. eq. of BrO3-
nSeO32- × 2 = 10-3 × 5;
nSeO32- = 2.5 × 10-3
(II)
In reaction (II)
gm.eq. of BrO3- = gm.eq.of AsO2-
nBrO3- × 6 = nAsO2- × 2
=
× 2 = 10-3nBrO3- =
In reaction (I)
moles of BrO3- consumed
= 10-3gm eq. of SeO32- = gm. eq. of BrO3-
nSeO32- × 2 = 10-3 × 5;
nSeO32- = 2.5 × 10-3
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