CHEM Osmometry

Question 1

Describe the four colligative properties

Answer 1

Increased osmolality:
1. Increases boiling point
2. Decreases vapour pressure
3. Increases osmotic pressure
4. Decreases freezing point depression

Question 1

Differentiate Molarity vs Molality; Osmolality vs Osmolarity

Answer 1

Molarity: mol/ L
Molality: mol/ kg

Osmolarity: number of particles/ L
Osmolality: number of particles/ kg

Question 2

Describe the mathematical relationship between freezing point and osmolarity

Answer 2

• “freezing point depression”
• freezing point of a solution decreases by -1.86°C for each Osm of particles/kg

[(1000 mOsm/kg)/ -1.86°C] = [(x mOsm/kg)/ measured freezing point of sample]

Question 3

Describe the steps of freezing point depression measurements

Answer 3

• 20 μl of sample is placed in cooling chamber via syringe injection
• “supercooling” occurs; temp drops below expected freezing point (-7°C)
• “seeding”; a physical shock initiates formation of ice crystals
• sample begins to solidify and releases heat
• thermistor detects decrease in resistance as temp increases
• true freezing point (mOsm/kg) = where temp plateaus

Question 4

Identify the 4 parts of a freezing point depression osmometer

Answer 4

1. Cooling mechanism
2. Thermistor; measures temp
3. Mechanism for “seeding”
4. Display

Question 5

Discuss calibration of a freezing point osmometer

Answer 5

• calibrated using NaCl of known concentrations
• must be performed at same temp as patient samples

Question 6

Ref: Serum Osm

Answer 6

280 - 300 mOsm/kg

Question 7

Ref: Osmol Gap

Answer 7

<10 mmol/kg

Question 8

What is an osmole (Osm) ?

Answer 8

The amount of solute that dissociates to produce 1 mole of particles in a solution

NOTE: osmolality is per kg of water

Question 9

Osm of NaCl

Answer 9

2 Osm; NaCl dissociates into individual ions

Question 10

Osm of CH4N2O

Answer 10

1 Osm; urea does not dissociate

Question 11

Define osmotic pressure

Answer 11

The hydrostatic pressure caused by a difference in the amounts of solutes between solutions that are separated by a semipermeable membrane

Question 12

Vapour pressure is measured __ by measuring the __ of a solution; the temperature at which condensation of water from a vapour state occurs.

Answer 12

Vapour pressure is measured INDIRECTLY by measuring the DEW POINT of a solution; the temperature at which condensation of water from a vapour state occurs.

Question 13

Why is freezing point depression the most common method to measure osmolality ?

Answer 13

• detects volatile substances in biological substances; does not denature proteins
• not sensitive to variation in atmospheric temperature

Question 14

Sources of Error in determining Freezing point

Answer 14

• variation in pipetting sample volumes
• air bubbles in sample
• preparing sample too early and evaporation occurs
• contamination can initiate seeding = inaccurate results

Question 15

Formula for Osmolal Gap

Answer 15

Osmolal gap (mOsm/kg) = Measured osmolality (mOsm/kg) - Calculated osmolality (mmol/L)

Question 16

The osmolal gap is used to screen for exogenous substances such as __, __, __ and __.

Answer 16

The osmolal gap is used to screen for exogenous substances such as ETHANOL, METHANOL, ISOPROPANOL and ETHYLENE GLYCOL.

Question 17

Formula for Calculated Osmolality

Answer 17

[2Na+] + [glucose] + [urea] = mmol/L

Question 18

List some clinical utility of osmometry

Answer 18

• evaluate ability of kidney to concentrate urine
• monitor kidney disease
• monitor fluid and electrolytes therapy
• aid in diagnosis of hypernatremia and hyponatremia
• evaluate secretion and renal response of ADH

Question 19

Which of the following can be assessed by measuring urine osmolality?

a.
filtering capacity of glomeruli

b.
renal blood flow

c.
renal concentrating ability

d.
renal tubular secretion

Answer 19

c.
renal concentrating ability

Question 20

Which of the following is an advantage of osmometry when compared to specific gravity?

a.
each solute contribution is proportional to its molecular weight

b.
all solutes contribute equally regardless of molecular weight

c.
only the contribution of ionic solutes is measured

d.
only the contribution of non-ionic substances is measured

Answer 20

b.
all solutes contribute equally regardless of molecular weight

Question 21

What component in an osmometer that measures the heat of fusion?

a.
barometer

b.
thermometer

c.
thermistor

d.
manometer

Answer 21

c.
thermistor

Question 22

What is the mOsm equivalent of 2 mol of sodium chloride in 1kg of water?

a. 1

b. 2

c. 2000

d. 4000

Answer 22

d. 4000

Question 23

Which of the following warms the sample in a freeze point osmometer?

a.
heat of vaporization

b.
heat of fusion

c.
heat of condensation

d.
heat of sublimation

Answer 23

b.
heat of fusion

NOTE: the amount of energy required to convert one mole of solid into liquid

Question 24

Calculate the change in freezing point (°C) when 1g of sodium chloride is added to 1kg of water?

a.
-1.86°C

b.
+1.86°C

c.
-3.72°C

d.
-0.93°C

Answer 24

c.
-3.72°C

NOTE:
[(1000 mOsm/kg)/ -1.86°C] = [(x mOsm/kg)/ measured freezing point of sample]

Question 25

Which of the following can falsely decrease osmolality reading when using a freezing point osmometer?

a.
ethylene glycol

b.
dust

c.
serum stored at room temperature

d.
hemolysis

Answer 25

b.
dust

NOTE: particulate matter can induce ice seeding prematurely = falsely decreased

Question 26

Which of the following can be assessed by measuring urine osmolality?

a.
glomerular filtration

b.
renal tubular secretion

c.
renal blood flow

d.
renal tubular function

Answer 26

d.
renal tubular function; ability to concentrate urine

NOTE: glomerular filtration is measured by clearance