Quote from GingerSue
the formula used to convert millimoles to milliequivalents is

mEq = mmol/L X valence

How is this used when considering sodium?

Sodium's atomic weight is 23 mg,

therefore 23 mg of sodium is 1 mmol of sodium

a mmol of a substance is the atomic weight in milligrams

http://www.merck.com/mmpe/appendixes/ap1/ap1a.html
"The following can be used to convert between mEq, mg, and mmol:

mEq = mg/formula wt × valence = mmol × valence

mg = mEq × formula wt /valence = mmol × formula wt

mmol = mg/formula wt = mEq/valence

(Note: Formula wt = atomic or molecular wt.)"

So, for Sodium:

1 mEq = 23mg/23 (formula wt) x 1 (valence) = 1mmol x 1 (valence)

23mg = 1mEq x 23 (formula wt) /1 (valence) = 1mmol x 23 (formula wt)

1mmol = 23mg/23 (formula wt) = 1mEq/1 (valence)

Calcium has a formula weight of 40, and a valence of 2, so:

2 mEq = 40mg/40 (formula wt) x 2 (valence) = 1mmol x 2 (valence)

20mg = 1mEq x 40 (formula wt) /2 (valence) = 0.5mmol x 40 (formula wt)

1mmol = 40mg/40 (formula wt) = 1mEq/2 (valence)

40mg of Ca = 2 mEq

1mmol of Ca = 0.5 mEq

1 mEq of Ca = 20mg