Question

Consider the following reaction:

CaX + 2H⁺ ↔ H₂X + Ca²⁺

If 3 cmol_c of adsorbed Ca²⁺ ions per kilogram of soil were replaced by H⁺ ions, how many grams of Ca²⁺ ions would be replaced? (Note: 3 cmol of positive charge (not Ca²⁺ ions) are replaced; 1 mol = 100 cmol).

Answer 1

Given that there are 3 cmol of adsorbed Ca²⁺ ions are there per kilogram of soil.

Given that 100 cmol = 1 mol

Then 3 cmol = ?

Number of moles of Ca⁺² ions = 3/100 = 0.03 moles

This means a 3 cmol of adsorbed Ca⁺² ions means 0.03 moles of calcium ions are adsorbed on kilogram of soil.

We have the formula to calculate number of moles

Number of moles = (Weight/Molar mass)

We need to calculate the weight of calcium ions corresponding to 0.03 moles

We have the molar mass of Ca = 40.078 g/mol

Ca and Ca⁺² have the same molar mass as the weight do not change with change in electrons.

Therefore from the above formula

0.03 mol = (Weight of Calcium ions in gms / 40.078 g/mol)  

Weight of calcium ions = 0.03 moles x 40.078 g/mol = 1.20234 gms

Therefore 1.20234 grams of Calcium ions would be replaced for one kilogram of soil.