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Learn how to use three lab values (Sodium, glucose, and BUN) to approximate your plasma osmolarity. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

## Want to join the conversation?

• What does calculating the osmolarity for yourself or your patient really tell you about your/their health?
• According to Wikpedia, "Plasma osmolarity measures the body's electrolyte-water balance."
An increase in plasma osmolarity can be a sign of dehydration or disease. A significant decrease often signifies other medical issues. Plasma osmolarity generally influences the passage of water in and out of cell membranes. When osmolarity increases, Antidiuretic Hormone (ADH) is secreted. It triggers the body to re-absorb water.
• How do you know if a molecule will behave like one molecule or will split up when in water?
• Ionic compounds (like table salt) will mostly tend to dissociate (or break up) in water, and covalent compounds (like glucose) generally don't. Remembering that and looking at the molecule will usually help you get an estimate of the osmolarity. For more precise calculations, you'd need to look up the van 't Hoff factor for the molecule, which is a number that tells you how many osmoles you get per mole.
• Doesn't the milligram of the of both the glucose and blood urea nitrogen have to be converted from mg to grams? AMU is g/mol...
• That's why we are talking about millimoles here. So if 1 mole of glucose = 180 g, then 1 mmol = 180 mg. You divide both sides by 1000.
• How do you determine the increasing osmolarity of 0.15 M C2H6, 0.15 M BaCl2, 0.15 M NaI?
• Osmolarity of 0.15 M C2H6 is 0.15.
Osmolarity of 0.15 M NaCl is 0.3 (sodium chloride will dissociate to give two ions: Na+ and Cl-). Osmolarity of 0.15 M BaCl2 is 0.45 (BaCl2 will dissociate to give 3 ions: Ba2+ and two Cl-).
• How can i find the element from its mass?
• You can find the element from the periodic table. For example, an element has mass 16. In the periodic table you look for an element that has the mass 16 and it is oxygen.
• molarity, normality, molal
• M polarity is concentration mol/L morality is concentration mold/kg and a mole is a base line for weight determination of elements
• What if I have an organic dye (let's say carboxyfluorescein) and I have to add NaOH to dissolve it in water and after dissolution, I see that the pH is very close to 7 implying that there is probably no excess OH-, do I include only Na+ to calculate my osmolarity or do I need to include the doubly negatively charged dye as well when counting species? What I mean is, does size of the counter ion of Na+ matter when calculating osmolarity? does the doubly negatively charged dye molecule contribute the same osmolarity as Cl-?