How to use Ksp to Compare Solubility

Let's consider the solubility product (Ksp) expressions of these 2 sparingly soluble salts, AgBr and Ag2CO3.

For AgBr, Ksp = [Ag+][Br-]

For Ag2CO3, Ksp = [Ag+]2[CO32-]

Since Ksp is related to concentration of ions in a saturated solution, can we use Ksp to compare solubility of 2 salts directly?

Let's take a look at the solubility product (Ksp) and solubility of these 2 salts.

230116 compare Ksp and solubility

Notice Ag2CO3 has a slightly higher Ksp but significantly larger solubility.

The reason for the disproportionate difference in Ksp and solubility is due to the mathematical conversion from Ksp to solubility.

For AgBr with 2 ions dissociated, the conversion from Ksp to solubility is by square root.

230116 calculate AgBr solubility

For Ag2CO3 with 3 ions dissociated, the conversion from Ksp to solubility is by cube root.

230116 calculate Ag2CO3 solubility

Due to this disproportionate difference, there is a possibility that a salt can have a bigger Ksp but lower solubility as compared to another salt.

230116 compare Ksp and solubility 2

Notice the 2 highlighted salts, AgCl has a bigger Ksp but is less soluble than Ag2CrO4.

Hence when comparing solubility of 2 salts that dissociate to give the same number of ions, we can use Ksp to compare solubility directly.

But when the number of ions dissociated is different, we have to calculate the solubility from Ksp to determine which salt is more soluble.

Topic: Solubility Product, Physical Chemistry, A Level Chemistry, Singapore

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