Question

How do you calculate percent dissociation?

Answer 1

Hint: Percent dissociation is going to be denoted with a symbol $\alpha$ (alpha). Dissociation is nothing but the conversion of a compound into its respective ions in a suitable solvent. Percent of dissociation is of a compound is going to vary from solvent to solvent.

Complete step by step solution:
- In the question it is given how to calculate the percent dissociation.
- There is a way to calculate the percentage of dissociation of a compound, we have to divide the mass of the dissociated ions with total mass of the dissociated and not dissociated compound and multiply with 100 to get in percentage.
- Generally all the compounds won't be soluble in all solvents.
- Few compounds are going to be soluble in the form of converting into respective ions by dissociation.
- If we are going to calculate the percent of dissociation of an acid then it is called acid dissociation and if we are going to calculate the percent dissociation of a base then it is called base percent of dissociation.
- Percent dissociation is going to depend on the concentration of the acid or base or neutral compound.
For example here we can discuss the calculation of percentage of dissociation of 0.1 M solution of acetic acid ($K_a={10}^{-5}$ ).
- Here $K_a$ is the dissociation constant of the acid HA and $\alpha$ is the degree of dissociation.
- The formula to calculate the dissociation constant is as follows.
$$\alpha ={\displaystyle \frac{(C_{\alpha })(C_{\alpha })}{(1-\alpha )}}={\displaystyle \frac{{(C_{\alpha })}^2}{1-\alpha }}$$
If $\alpha <<1$ then we can write the above equation as follows.
$$\alpha =\sqrt{{\displaystyle \frac{K_a}{C}}}$$
- Substitute the known values in the above formula to get the dissociation value of 0.1 M acetic acid and it is as follows.
$$\alpha =\sqrt{{\displaystyle \frac{K_a}{C}}}=\sqrt{{\displaystyle \frac{{10}^{-5}}{0.1}}}={10}^{-2}=0.01\text{ \% }$$

Note: The percent of dissociation of an acid or a base is going to depend on their strength. If the acid is too strong then it will have high percent of dissociation and if the acid is weak it will have less percent of dissociation. Like the same manner percent of dissociation also going to be applicable to the strength of the base. For the chemicals like benzene, acetonitrile we cannot calculate the dissociation constant.