Question

Degree of ionisation is equal to :
A.Total number of moles of the electrolyte present in solution
B.Total number of moles of the electrolyte dissociated into ions
C.Number of moles dissociated / Total number of moles dissolved
D.Total number of moles dissolved / Number of moles dissociated

Answer 1

Hint: Ionisation refers to the process whereby an atom or molecule loses one or several electrons from its atomic orbital (positively charged ion ) , or conversely gains an additional one , from an incoming free electron (negatively charged ion ) .

Complete step by step answer:
The degree of ionisation or dissociation is defined as the fraction of the total substance that undergoes dissociation . It is represented by $\alpha $ .
\[degree{\text{ }}of{\text{ }}dissociation{\text{ }} = \dfrac{{number{\text{ }}of{\text{ }}moles{\text{ }}dissociated}}{{total{\text{ }}number{\text{ }}of{\text{ }}moles{\text{ }}taken}}\]

So, the correct answer is Option C .

Additional Information:
When ionic compounds are dissolved in aqueous solution , they dissociate to give ions . Therefore the number of particles increases and hence colligative properties changes as they depend on the number of particles . This is where the concept of abnormal molecular mass comes into play .
When the molecular mass of a substance determined by studying any of the colligative properties comes out to be different than the theoretical value , the substance is said to have abnormal molecular mass .
The value of degree of dissociation is used to calculate the value of the van't Hoff factor which in turn is used to calculate the abnormal molecular mass .

Note:
The degree of dissociation is a measure of the strength of an acid . Therefore the dissociation constant of a weak acid is very small , almost negligible . By looking at the value of degree of ionisation we can tell whether the acid is strong or weak .( weak acids have large value of degree of ionisation )