Question

Equivalent weight of potash alum is $[{K_2}S{O_4}.A{l_2}{(S{O_4})_3}.24{H_2}O]$.
A. M/2
B. M/3
C. M/8
D. M/4

Answer 1

To find out the equivalent weight of the potash alum, the total charge of the compound is needed to be calculated. The water is a neutral compound so it does not have any charge.

Complete answer: The potash alum is a double salt also known as potassium aluminium sulphate or potassium aluminium. The chemical formula is $[{K_2}S{O_4}.A{l_2}{(S{O_4})_3}.24{H_2}O]$.
The equivalent weight is calculated by dividing the molecular weight of the compound by its n-factor.
The formula for the calculation of equivalent weight is shown below.
$E.W = \dfrac{M}{n}$
Where,
E.W is the equivalent weight.
M is the molecular weight.
n is the number of charge.
To calculate the equivalent weight of potash alum, first we need to find the number of charge present in the compound.
$[K_2^{ + 2}SO_4^{ - 2}.Al_2^{ + 6}(S{O_4})_3^{ - 6}.24{H_2}O]$
Charge of potassium is +1 as two potassium ion is present total charge is +2, the charge on sulphate ion is -2, the charge on aluminium is +3 as two aluminium ion is present the total charge is +6. In sulphate ion the charge is -2, three sulphate ion is present the total charge is -6. Water molecule is neutral, therefore, it has no charge.
The total number of cation is +8 and the total number of anion is -8.
Therefore, the value of n is 8.
Thus, the equivalent weight of potash alum is given as shown below.
$E.W = \dfrac{M}{8}$
Therefore, the correct option is C.

Note: Make sure to multiply the charge with the total number of atoms to get total charge. For an acidic compound the molecular weight is divided by the basicity value, for a basic compound the molecular weight is divided by the acidity value, for the salts the molecular weight is divided by the number of charges.