Chemistry: Agricultural lime: chemical ratio of calcite to dolomite

Self-tutoring about the chemical composition of an agricultural lime product: the tutor makes a calculation.

Dolomite, apparently, is CaMg(CO₃)₂. Calcite, on the other hand, is CaCO₃. Moreover, the mass of magnesium (Mg) is 24, while calcium (Ca) is 40.

I recently looked at a bag of agricultural lime that said its typical analysis is 26% calcium, 6% magnesium. The question: how much of it is dolomite, vs calcite, assuming the whole sample is some mix of them?

Chemical analysis, unless otherwise stated, is done by mass: therefore, the ratio of Ca to Mg is 26 to 6 by mass.

On the other hand, molecular formulas are by number (moles) of each atom present in a given unit.

Formulaically, the Ca:calcite ratio is 1:1, as is the Ca:dolomite; the Mg:calcite is 0, while Mg:dolomite is 1:1.

The equation to solve, if x is moles dolomite while y is moles calcite:

(40x + 40y)/24x = 26/6

Then, cross-multiplying,

240x + 240y = 624x

240y = 384x

y/x = 384/240

Therefore, it seems that the molar ratio of calcite to dolomite is 1.6/1, or 8/5.

Source:

Calcite-Dolomite

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