By Riti Gupta
Updated: March 13, 2025 11:55 pm EST
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In laboratory work, solution concentrations are most commonly expressed in molarity (moles per liter). However, many protocols and catalogues list concentrations as milligrams or grams of solute per liter (mg/L or g/L). Converting mg/L to molarity is straightforward once you know the molar mass of the solute. Below is a clear, step‑by‑step method to perform this conversion.
Because molar mass is expressed in grams per mole, the first conversion is always to grams. For example, if a solution contains 1,567 mg of NaCl in 1 L, multiply by the factor 1 g / 1,000 mg:
1,567 mg × (1 g / 1,000 mg) = 1.567 g
This simple unit‑cancellation confirms the correctness of the conversion.
The molar mass of a compound is the sum of the atomic weights of its constituent atoms. Sodium chloride, for instance, has a molar mass of 22.99 g/mol (Na) + 35.45 g/mol (Cl) = 58.44 g/mol. Using this value, convert the grams of NaCl to moles:
1.567 g NaCl × (1 mol NaCl / 58.44 g NaCl) = 0.027 mol NaCl
Finally, divide by the solution volume (1 L) to obtain the molarity:
0.027 mol / 1 L = 0.027 M NaCl
Because 1 mg/mL equals 1 g/L, you can skip the milligram‑to‑gram step when working with mg/mL. For example, a 15 mg/mL solution of magnesium chloride (MgCl₂) has a concentration of 15 g/L. Using the molar mass of MgCl₂ (95.21 g/mol), the molarity is:
15 g / 1 L × (1 mol / 95.21 g) = 0.16 M MgCl₂
For macromolecules such as proteins, the same approach applies. Proteinase K has a molecular weight of approximately 29,800 g/mol. A 25 mg/mL (25 g/L) solution yields:
25 g / 1 L × (1 mol / 29,800 g) = 8.4 × 10⁻⁴ M Proteinase K
These examples illustrate how mg/mL values translate to molarity, providing a useful reference for both small molecules and proteins.
