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In chemistry, concentration describes how much of a substance is present per unit volume. While concentration can refer to mass, it also applies to any quantifiable entity—gas molecules, ions, or even photons—within a solution.
When a solid dissolves in a liquid, the resulting solution’s concentration can be expressed in multiple ways. Chemical reactions depend on the ratio of reacting particles, not merely their mass, making the concepts of moles, equivalents, and milliequivalents essential—especially in medicine and clinical pharmacology.
Consider a simple reaction: one potassium (K) atom reacts with one chlorine (Cl) atom to form potassium chloride (KCl) with no excess atoms. This 1:1 ratio is due to stoichiometry, not equal masses.
A mole contains 6.02 × 10²³ entities—atoms or molecules. Every element’s molar mass (the mass of one mole in grams) is listed in the periodic table; for instance, carbon’s molar mass is 12.011 g/mol.
Molar masses vary widely across the periodic table, from hydrogen (1.008 g/mol) to uranium (238.028 g/mol), reflecting the increasing number of protons and neutrons in heavier elements.
When a solute dissolves, it can dissociate into multiple ions, each contributing to the solution’s electrical charge. The valence of a solute quantifies this: one KCl molecule dissociates into two ions (K⁺ and Cl⁻), giving KCl a valence of 2.
Similarly, calcium chloride (CaCl₂) dissociates into three ions (one Ca²⁺ and two Cl⁻), so its valence is 3. The equivalent—and its millimolar counterpart, the milliequivalent (mEq)—accounts for both the mass of the solute and its valence:
(mEq = (mass × valence) / MW)
Here, mass and the molar mass (MW) are expressed in milligrams. Concentration in equivalents per liter is a standard unit, commonly written as mEq/L.
1. Potassium in 750 mL of solution – A K⁺ concentration of 58.65 mg/L with a molar mass of 39.1 g/mol (39,100 mg/mol).
First, compute the total mass of potassium: 58.65 mg/L × 0.75 L = 43.9875 mg.
With a valence of 1, mEq = (43.9875 mg × 1) / 39,100 mg/mol = 0.001125 mol ≈ 1.13 mEq.
2. NaCl in 400 mL of solution – 30 mg of NaCl per 400 mL, molecular weight 58.44 g/mol (58,440 mg/mol).
NaCl dissociates into two ions, so valence = 2. mEq = (30 mg × 2) / 58,440 mg/mol = 0.001026 mol ≈ 1.03 mEq.
To express this as a concentration: 1.03 mEq / 0.4 L = 2.575 mEq/L.
