What is the mEq/ml (Milliequivalents) of 50% Magnesium Sulfate? (Molecular weight of MgSO4 is 120.4g/mol.)

A 50% Magnesium Sulfate (MgSO₄) solution for injection is prepared using magnesium sulfate heptahydrate (MgSO₄·7H₂O). This concentration corresponds to 500 mg of MgSO₄·7H₂O per mL. The molecular weight (MW) of the heptahydrate form is approximately 246.47 g/mol. Magnesium is a divalent cation (Mg²⁺), so its valence is 2. The formula for milliequivalents (mEq) is: mEq/mL = (mg/mL × valence) / MW Applying the values: mEq/mL = (500 mg/mL × 2) / 246.47 g/mol ≈ 4.06 mEq/mL. This value is closest to 4.11 mEq/mL. The molecular weight of anhydrous MgSO₄ (120.4 g/mol) provided in the question is a common distractor, as the clinically used product is the hydrated form.

A. 8.3 mEq/ml: This is the incorrect result from using the provided anhydrous molecular weight (120.4 g/mol) instead of the correct heptahydrate molecular weight.
C. 62 mEq/ml: This value is numerically incorrect and does not result from any logical application of the mEq formula to the given data.
D. 50 mEq/ml: This incorrectly equates the percentage strength of the solution (50%) with the milliequivalent concentration.
E. 2.06 mEq/ml: This is approximately half the correct value, which would result from incorrectly using a valence of 1 instead of 2 for magnesium.

1. U.S. National Library of Medicine, DailyMed. Label: MAGNESIUM SULFATE- magnesium sulfate injection, solution. "Each mL contains: Magnesium Sulfate (Heptahydrate) 500 mg...which provides 4.06 mEq of magnesium." Document ID: 2d120533-0a30-4155-9a4e-551223244b78. Retrieved from https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=2d120533-0a30-4155-9a4e-551223244b78

2. Stoklosa, M. J., & Ansel, H. C. (2012). Pharmaceutical Calculations (14th ed.). Lippincott Williams & Wilkins. Chapter 9, "Electrolyte and Intravenous Solutions," provides the foundational formulas for calculating milliequivalents, confirming mEq = (mg × valence) / MW.

3. University of Kentucky College of Pharmacy. Pharmaceutical Calculations Workbook. Page 51, Problem 10 and its solution demonstrate the standard calculation for a 50% magnesium sulfate solution using the molecular weight of the heptahydrate form (246.5 g/mol), yielding a result of 4.06 mEq/mL.