H2 Chemistry 10 - Chemistry of Aqueous Solutions: Solubility Product and Precipitation

Step 1 - State the problem

The solubility product of barium sulfate is one point one times ten to the minus ten moles squared per decimetre to the sixth at twenty-five degrees Celsius.

Step 1 - State the problem

Will a precipitate form when twenty-five point zero cubic centimetres of zero point zero one zero molar barium chloride is mixed with twenty-five point zero cubic centimetres of zero point zero two zero molar sodium sulfate?

Step 1 - State the problem

We need to calculate the ionic product and compare it with K s p.

Step 2 - Calculate concentrations after mixing

When the two solutions are mixed, the total volume doubles to fifty point zero cubic centimetres.

Step 2 - Calculate concentrations after mixing

The concentration of barium ions is halved because the volume doubles: zero point zero one zero divided by two gives zero point zero zero five zero molar.

Step 2 - Calculate concentrations after mixing

The concentration of sulfate ions is also halved: zero point zero two zero divided by two gives zero point zero one zero molar.

Step 3 - Calculate the ionic product

The ionic product, Q, is calculated using the same expression as K s p but with the actual concentrations in the mixture.

Step 3 - Calculate the ionic product

Q equals zero point zero zero five zero times zero point zero one zero, which equals five point zero times ten to the minus five.

Step 3 - Calculate the ionic product

This is much larger than K s p, which is one point one times ten to the minus ten.

Step 4 - Compare Q with Ksp and conclude

Since Q is five point zero times ten to the minus five and K s p is one point one times ten to the minus ten, Q is much greater than K s p.

Step 4 - Compare Q with Ksp and conclude

This means the solution is supersaturated with respect to barium sulfate, so a white precipitate of barium sulfate will form.

Step 4 - Compare Q with Ksp and conclude

Precipitation continues until Q falls back to equal K s p.

Step 5 - Summary and common pitfalls

To decide whether a precipitate forms, always compare the ionic product Q with K s p.

Step 5 - Summary and common pitfalls

If Q is greater than K s p, precipitation occurs. If Q is less than K s p, no precipitate forms. If Q equals K s p, the solution is just saturated.

Step 5 - Summary and common pitfalls

Common mistake: forgetting to account for dilution when two solutions are mixed.

Step 5 - Summary and common pitfalls

Always calculate the new concentrations in the total mixed volume before finding Q.