[rjordan393]

Is there an equation to determine the amount of Calcium & Magnesium from calcium and magnesium carbonate in parts per million.

[Dan]

You mean, for example, what is the concentration of Ca in solid CaCO₃ in ppm?

Of course you can do it, but mass fraction (% wt) would be an easier number to handle (i.e. a more sensible format to report in).

Can you determine the % wt?

Hint: Molecular mass and atomic mass

[rjordan393]

I am just a fish keeper doing water analysis with test kits. I do have two equations but I am not sure they are correct. So when my results are in calcium carbonate or magnesium carbonate, I just need a simple equation to determine their elementary forms. The equation I have on hand is:
CaCO3 divided by 2.5 = Ca.
MgCO3 divided by 4.1 = Mg.  Are these correct?

[Borek]

CaCO3 divided by 2.5 = Ca.

Reasonably correct.

MgCO3 divided by 4.1 = Mg

This one is wrong.

As Dan suggested, you can easily do the conversion using molar masses and atomic masses.

Molar mass of CaCO₃ is 100 g/mol (40 g/mol for Ca, 12 g/mol for C, and 3*16 g/mol for oxygen). As the molar mass of calcium is 40 g/mol, the mass fraction of calcium in calcium carbonate is 40/100 = 0.4, multiplying by 0.4 is equivalent to dividing by 2.5.

Molar mass of Mg is 24 g/mol.

[rjordan393]

I believe I am getting a handle on it. People of science when speaking of the general hardness of tap water always express both concentrations of calcium carbonate and magnesium carbonate together as CaCO3. I know there are other things that contribute to hardness but not much under ordinary circumstances.
I will consult a molar mass calculator and base the equations on it. Thanks.

[Hunter2]

No. If you want to have Magnesium then you have to use everything containing Magnesium. Molar mass of Magnesiucarbonate is 84 g/mol.

[Borek]

People of science when speaking of the general hardness of tap water always express both concentrations of calcium carbonate and magnesium carbonate together as CaCO3.

Always is perhaps an exaggeration, but yes, it is not unusual to express water hardness using this type of units. Note that once you have the hardness listed this way you can't convert it back to individual concentrations/fractions of Ca and Mg.

[rjordan393]

One final question: Why is MgCO3 expressed as CaCO3 when testing? If someone is doing calculations, a mix up can occur easily between MgCO3 & CaCO3.

[Borek]

You need a way of comparing different samples of hard water. For most practical purposes calcium and magnesium behave identically, so we need some kind of sum. Sum of masses would be ambiguous, and converting to one compound makes the number much more reliable. As of today it would be much better to express hardness in terms of moles (millimoles) of calcium and magnesium present, but for historical reasons we use the old scale, which predates definitions of moles and equivalents.