Question

What is the n-factor for potash alum?

Answer 1

Hint: In order to calculate n-factor for potash alum (a salt), first write its molecular formula. After writing its molecular formula, determine the total number of positive or negative charges which will give n-factor for the potash alum.

Step-by-step answer:
The n-factor for any particular salt is equivalent to the number of positive or negative charges present on it. We know that the molecular formula of potash alum is $${{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} \cdot {\text{A}}{{\text{l}}_{\text{2}}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}} \cdot 24{{\text{H}}_{\text{2}}}{\text{O}}$$.

From the dissociation of potash alum, we will be able to know the number of positive and negative charges present in potash alum. Thus the dissociation of potash alum is as follows:

$${{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} \cdot {\text{A}}{{\text{l}}_{\text{2}}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}} \cdot 24{{\text{H}}_{\text{2}}}{\text{O}} \to 2{{\text{K}}^ + } + 2{\text{A}}{{\text{l}}^{3 + }} + 4{\text{SO}}_4^{2 - } + 24{{\text{H}}_{\text{2}}}{\text{O}}$$

On dissociation, two potassium ions (positive charges), two aluminium ions (positive charges) and four sulphate ions (negative charges) are produced. n-factor can be obtained from the total number of positive charges or total number of negative charges present in potash alum.

Since we know that charge on potassium (K) is +1 and charge on aluminium (Al) is +3. Thus total number of positive charges can be calculated as follows:

$$\displaylines{
  {\text{Total}}\;{\text{positive}}\;{\text{charges}} = 2\left( {{\text{charge}}\;{\text{of}}\;{{\text{K}}^ + }} \right) + 2\left( {{\text{charge}}\;{\text{of}}\;{\text{A}}{{\text{l}}^{3 + }}} \right) \cr
   = 2 \times 1 + 2 \times 3 \cr
   = 8 \cr} $$

The total number positive charges is equal to 8. Thus it can be said that n-factor for potash alum is 8.

Since n-factor for a salt can also be calculated from total negative charges. The total charge on sulphate ion $$\left( {{\text{SO}}_4^{2 - }} \right)$$ is equal to $$ - 2$$. Hence total number of negative charges can be calculated as follows:

$$\displaylines{
  {\text{Total}}\;{\text{negative}}\;{\text{charges}} = 4\left( {{\text{charge}}\;{\text{on}}\;{\text{SO}}_4^{2 - }} \right) \cr
   = 4 \times 2 \cr
   = 8 \cr} $$

The total negative charges is equal to 8. Thus it can be said that n-factor for potash alum is 8.

Note: To calculate n-factor for salts, check total number of positive or total charges on salt. Do not consider water while calculating n-factor for salts.