Page 41 - Chemistry - XI

(ii) It should be readily soluble in water.

(iii) It should be neither hygroscopes nor effl orescent.
(iv) It should be stable and unaff ected by air.
(v) Its equivalent weight should be large so that there are minimum errors in weighing.

(vi) Its composition should not change on standing during storage.

COOH

Hydrated oxalic acid | .2 HO , Mohr’s salt (FeSO .(NH ) SO .6H O), anhydrous sodium

4
4 2
4
2
2
COOH

carbonate (Na CO ), etc. are primary standards.
2
3
12. Secondary standard is a substance that does not fulfi l the above requirements and its standard solution
cannot be prepared directly by weighing.
The exact strength of such solution is found by titrating its solution of approximate strength against a
standard solution of the suitable reagent. This process is called standardisation. Hydrocholoric acid,
acetic acid, sodium hydroxide, potassium permanganate, etc. are secondary standards.
Formula Used in Calculations in Volumetric Analysis
1. Normality equation:
N V = N V 2
1
1
2
From here, knowing the normality of one solution, that of the other solution can be calculated.
2. Relation between strength and normality:
Strength = Normality × Eq. mass

3. Molarity equation:

MV MV
1 1 = 2 2
n n
1 2
Molarity of solutionA Volume of solutionA Molarity of solutionB Volume of solutionB

.
No of molesofA in balanceed equation No of mole. s sofB in balanced equation
e.g. Reaction involved in titration of KMnO with oxalic acid is:
4
COOH

2KMnO 3HSO 5 | KSO 2MnSO 10CO 8H O
4 2 4 2 4 4 2 2
COOH
MV MV MV n
1 1 = 2 2 or 1 1 = 1
n n MV n
1 2 2 2 2
( KMnO ) ( Oxalic acid)
4
MV MV MV 2
1 1 = 2 2 or 1 1 =
2 5 MV 5
2 2
4. Relation between molarity and strength:

Strength = Molarity × Molecular mass

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