Chemistry Laboratory 1

The osmolality of a urine sample or serum specimen is measured by a change in the
freezing point
Osmolal gap is
the difference between the calculated and measured osmolality value
The formula for calculating serum osmolality that incorporates a correction for the water content of plasma is
(2 Na + Glucose / 18) + (BUN/2.8)
The osmolal gap is defined as measured Osm/kg minus the calculated Osm/kg. The average osmolal gap is near
zero
The solute that contributes the most to the total serum osmolality is
sodium
An adult diabetic with renal complications has the following results. calculate theoretical osmolality
Sodium = 133 mEq/L
BUN = 84mg/dL
Glucose = 486 mg/dL
Creatinine = 5 mg/dL
(2 Na + Glucose)/ 18 + BUN/2.8
2 (144) + 486/18 + 84/2.8
323 mOsm/kg
A patient had the following serum results, calculate osmolality:
Sodium = 140 mEq/L
Glucose = 95 mg/dL
BUN = 10 mg/dL
2(Na) + glucose/18 + BUN/2.8
2(140) + 95/18 + 10/2.8
289 mOsm/kg
List the 4 colligative properties of solution
Osmotic pressure, decreased vapor pressure, decreased freezing point, and increased boiling point
One mole (Avagadro’s number of molecules) of non-ionizing solute dissolved in one kilogram of water will ________ the freezing point by ______degrees C.
Decrease, 1.86
The concentrating abilities of the kidney are the best evaluated by
osmolality
The osmolality of a solution is dependent upon
the number of particles
An increase in the osmolar gap may often be associated with
ingestion of toxic substances, ingestion of alcohol, presence of ketones
List the four primary constituents found in serum that are measured by osmometry
Na+, Cl-. Glucose and BUN (urea)
Define osmometry
techniques to measure the concentration of the dissolved solute particles in solution that contributes to osmotic pressure
When referring to osmolality, what are the units measured?
mOsm/ kg
Explain the difference between osmolarity and osmolality
Both are measurements of concentration of solutes in a solution, but osmolarity is a measurement of the concentration per volume, which is temperature dependent. Osmolality is concentration relative to mass of solution and is temperature independent