3.5/6 Assumption rule, solubility equilibrium, common ion effect, precipitate predictions

What is the assumption rule

The assumption rule states that the change from reactant to product in a reaction may be ignored when the concentration of any reactant divided by the Keq is > 100.

This allows for the quadratic formula to be avoided.

What is the solubility equilibrium

When a solution has reached a saturated state and no more solute can be dissolved and remains solid at the bottom of the solution, the undissolved solid actually dissolves, but at the same rate that dissolved particles recrystalize into the solid.

The solubility equilibrium Ksp is a measure of how soluble a solute is. It is calculated the same way as any other k value, but because the reactant is an undissolved solid, it is not included and is therefore only a numerator of products.

The greater the Ksp value, the greater the solubility of the compound

How does the addition of more solid beyond the saturation affect the equilibrium

The addition of more solid beyond the saturation does not affect the equilibrium

What is molar solubility

Molar solubility is the number of moles of solute dissolved in 1 L of solution.

What is the common ion effect

The common ion effect occurs when a solute is dissolved in a solution that contains the same ions as one of the dissolved products, causing the equilibrium to shift to the right and thus the speed of the forward rate of reaction to increase.

• For calculation purposes it just means you add the extra concentration of ions when calculating.

When mixing two solutions, how do you predict whether a precipitate will form

Step 1: write balanced chemical equation

Step 2: find least soluble product

Step 3: write dissolution for that product and determine the concentration of its dissolved ions

• concentration = [(m/L) x L] / (total liters)

Step 4: write the total ion product (TIP), which is essentially the Ksp formula but using those concentrations from step 3

Step 5: Compare the Ksp of the least soluble product to its TIP. If the TIP is greater, a precipitate will form; if they are equal or the TIP is lesser, no precipitate will form.