Milliequivalents: include charge as well as amount

For a monovalent ion such as sodium, 1 mmol corresponds to 1 mEq. For a divalent ion such as calcium, 1 mmol corresponds to 2 mEq. The charge magnitude is the multiplier. Do not use the signed charge to produce a negative dose amount.

A question about a salt must identify which ion is being counted. One mmol of sodium chloride corresponds to one mmol of sodium and one mmol of chloride. Do not add the two into a sodium amount. The ion of interest and its stoichiometry determine the conversion.

In a fictional exercise with sodium chloride molecular weight stated as 58.5 g/mol, 585 mg is 585/58.5 = 10 mmol of sodium chloride. Because each formula unit contains one sodium ion, it represents 10 mmol or 10 mEq of sodium. The same salt amount separately supplies 10 mmol of chloride.

Molecular weight in g/mol has the same numerical value as mg/mmol. That is why dividing mg by the stated molecular-weight number gives mmol. If you start with grams instead, convert to milligrams or maintain an explicitly consistent mol-to-mmol line. Do not mix the two silently.

For a fictional calcium chloride dihydrate exercise, use the supplied molecular weight of 147 g/mol. A 147 mg amount is 1 mmol of salt, which contains 1 mmol of calcium. Calcium has valence two, so that corresponds to 2 mEq of calcium. It is not 1 mEq simply because the salt amount is 1 mmol.

Using the molecular weight of an anhydrous salt for a hydrate gives the wrong amount. A formula with two ions of interest per unit would also need the corresponding factor. Read the full chemical name and supplied formula before selecting a molecular weight. This page does not recommend an electrolyte replacement dose.

If 1 mmol of a divalent ion represents 2 mEq, then 12 mEq of that ion represents 6 mmol. Check that the factor of two appears once, not twice. A common error is to multiply by valence while finding mmol and multiply by it again while finding mEq.

The linked academic electrolyte resource supports these definitions. The original examples are chemistry arithmetic, not a clinical infusion recipe. Actual electrolyte prescribing also depends on product labeling, patient status, route, concentration limits and monitoring. Use the question's given molecular weights and rounding instruction rather than substituting a remembered number.

Try it before reading the answer

Write your setup, units and check first. These are original practice exercises, not recalled exam items.

Continue learning