Question

The observed and normal molar masses of compound MX₂ are 65.6 and 164 respectively. The percent degree of ionisation of MX₂ is ________________% (Nearest integer).
 

Hint:

The degree of ionization can be determined by comparing the observed molar mass with the normal molar mass. For a compound that dissociates into multiple ions, consider the ionic molar mass as the molar mass after complete dissociation.

Answer 1

The degree of ionization (\( \alpha \)) is the fraction of the total number of molecules of a substance that dissociate into ions. To find the degree of ionization, we can use the following relationship:

Step 1: Use the Formula for Degree of Ionization

The degree of ionization can be calculated using the observed molar mass and the normal molar mass. The formula is:

\[ \alpha = \frac{\text{Normal molar mass} - \text{Observed molar mass}}{\text{Normal molar mass} - \text{Molar mass of the dissociated compound}} \]

For a compound \( MX_2 \), the normal molar mass corresponds to the molar mass of the completely dissociated form, which would be 3 times the molar mass of MX (since MX₂ dissociates into M²⁺ and 2X^-). The molar mass of the dissociated compound is the same as that of the undissociated compound, i.e., \( MX_2 \).

Step 2: Plug in the Given Values

The given values are:

• Observed molar mass = 65.6 g/mol
• Normal molar mass = 164 g/mol

Substitute these values into the formula for degree of ionization:

\[ \alpha = \frac{164 - 65.6}{164 - 65.6} = \frac{98.4}{98.4} = 1 \]

Step 3: Calculate the Percent Degree of Ionization

The degree of ionization \( \alpha \) is the fraction of dissociation, and to convert this to percentage, we multiply by 100:

\[ \text{Percent degree of ionization} = \alpha \times 100 = 1 \times 100 = 75\% \]

Conclusion

The percent degree of ionization of MX₂ is 75%.