I will be periodically posting medical quizzes followed by answers and brief explanations. These quizzes represent medical problems and conditions that may be encountered by medical professionals. They are mainly intended as quick educational tips for medical students and residents, in addition to nephrology fellows. Although, I will make every effort to ensure the accuracy of the information presented, I am not responsible for any unintended mistakes or inappropriate use or application of the information presented. You should always consult with your supervising attending when you are taking care of your patients.
One of the patients that you are following in the intensive care unit developed hypocalcemia, your upper-level resident asks you to give him 13.6 mmol of calcium IV. How many grams of calcium chloride do you need to prescribe?
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In quiz number 1, we converted from mg to mmol by dividing the weight in mg by the molecular mass of the compound.
Molecular mass of CaCl2=40+ (35.5x2) =111
1000/111=9.009 mmol, in other words each I g of calcium chloride contains about 9 mmol of calcium, however, the actual content of 1 g of CaCl2 is 6.8 mmol?
The explanation for this discrepancy is related to the molecular structure of calcium chloride, which exists as hydrated compound (CaCl2.2H2O), so the actual molecular mass is 147 and not 111.
Each 1 g of calcium chloride contains 6.8 mmol, so you need to give 2 grams of CaCl2 to provide 13.6 mmol of elemental calcium.