Question

The concentration of suphide ion in `0.1 M HCl` solution saturated with hydrogen sulphide is `1.0xx10^(-19)M`. If `10 mL` of this is added to `5 mL` of `0.04 M` solution of the following: `FeSO_(4), MnCl_(2), ZnCl_(z)` and `CdCl_(2)`. In which of these solutions precipitation will take place?

Answer 1

Correct Answer - Precipitation will take place in cadmium chloride solution.
For precipitation to take place, it is required that the calculated ionic product exceeds the `K_(sp)` value.
Before mixing:
`{:([S^(2-)]=1.0xx10^(-19)M,[M^(2+)]=0.04M),("volume"=10mL,"volume"=5mL):}`
After mixing:
`{:([S^(2-)]=?,[M^(2+)]=?),("volume"=(10+5)=15mL,"volume"=15mL):}`
`[S^(2-)] = (1.0 xx 10^(19) xx 10)/(15) = 6.67 xx 10^(-20) M`
`[M^(2+)] = (0.04 xx 5)/(15) = 1.33 xx 10^(-2) M`
Ionic product `= [M^(2+)][S^(2-)]`
`= (1.33 xx 10^(-2))(6.67 xx 10^(-20))`
`= 8.87 xx 10^(-22)`
This ionic product exceeds the `K_(sp)` of Zns and CdS. Therefore, precipitation will occur in `CdCl_(2)` and `ZnCl_(2)` solutions.