Calculate the grams of calcium carbonate (CaCO3) that will dissolve in 1.0L of 0.020 M Na2CO3?

Question

Calculate the grams of calcium carbonate (CaCO3) that will dissolve in 1.0L of 0.020 M Na2CO3?

Having trouble with this question. Any help would be appreciated!

Answer 1

The question must provide the temperature as the solubility depends on the temperature.

We will assume normal room temperature: 20 C

The pH is also very important as the solubility of CaCO3 decreases with pH.

First find the pH of the 0.02 M Na2CO3 using ICE table.

This yields pH = 11.3

At this pH, we can ignore the hydrolysis of CO3^2-.

Note that the solubility of CaCO3 will increase exponentially with decreasing pH

Solubility product of CaCO3, Ksp = 5 * 10^-9 at 20C (from literature)

Ksp = [Ca²⁺] [CO₃^2-] = 5*10^-9

Assume x moles/L of CaCO3 is dissolved at equilibrium (just before precipitation)

[Ca²⁺] = x mol/L

[CO₃^2-] = x + 0.02 mol/L (0.02 mol/L already present in solution)

x * (x+0.02) = 5*10^-9

Solving for x,

x = 2.5*10^(-7) mol/L

Mol wt of CaCO3 = 100

Solubility = 100 * 2.5*10^(-7) = 2.5*10^(-5) g/L

Mass of CaCO3 dissolved in 0.02M Na2CO3 = 2.5*10^-5 g