Every ion has an equivalent weight: its molar mass shared out over its charge, $\mathrm{EW} = \dfrac{M}{z}$ — read aloud: EW equals M over z. Calcium carbonate's is the trade's favourite number: M = 100.09 g/mol, z = 2, so EW = 50.04 — almost a round fifty. That near-roundness is exactly WHY chalk got elected the yardstick.

To restate any ion in chalk, scale by the yardstick over the ion's own equivalent: $C_{\mathrm{CaCO_3}} = C \cdot \dfrac{50.04}{\mathrm{EW}}$ — read aloud: C-as-chalk equals C times fifty-point-oh-four over EW. And lesson two's factors were never magic: 2.497 is 50.04/20.04 and 4.118 is 50.04/12.15, precomputed. The lighter the equivalent, the bigger the factor — light ions count for more chalk.