Question

Q10.An aqueous solution contains 5.00x10-2 mol/L of Ca2+ and 7.00x10-3 mol/L of SO4

2-

. Show whether

this solution would precipitate anhydrite CaSO4 or not provided that the Ksp for anhydrite is equal to 10-4.5.​

Answer 1

The precipitate will form.

Step-by-step explanation:

Let's write the equilibrium expression for the solubility product of calcium sulfate:

`CaSO_4(s)` ⇄
`Ca^(2+)(aq)+SO_4^(2-)(aq)`

The solubility product is defined as the product of the free ions raised to the power of their coefficients, in this case:

`K_(sp)=[Ca^(2+)][SO_4^(2-)]=10^(-4.5)`

Our idea is to find the solubility quotient, Q, and compare it to the K value. A precipitate will only form if Q > K. If Q < K, the precipitate won't form. In this case:

`Q_(sp)=[Ca^(2+)][SO_4^(2-)]=5.00\cdot10^(-2) M\cdot7.00\cdot10^(-3) M=3.5\cdot10^(-4)`

Now given the K value of:

`K_(sp)=10^(-4.5)=3.2\cdot10^(-5)`

Notice that:

`Q_(sp)>K_(sp)`

This means the precipitate will form, as we have an excess of free ions and the equilibrium will shift towards the formation of a precipitate to decrease the amount of free ions.