Sun28November0628PM 22

Measurement of the osmolality of biological fluids can provide useful diagnostic information.

Which one of the following physical principles is the most accurate and reliable method of measuring osmolality?

(Please select 1 option)

Lowering of vapour pressure

Depression of freezing point Correct

Elevation of boiling point

Depression of melting point

Change in osmotic pressure

Explanation

WIth the exception of depression of melting point all of the above are colligative properties.

Colligative properties are properties of solutions that depend on the number of dissolved particles in solution and not on the identities of the solutes.

An osmometer measures osmolality from the concentration of a substance in a solution. A freezing point osmometer determines concentration of a solution by measuring the freezing point of the solution, as depression of freezing point is directly correlated to concentration.

There are osmometers that measure concentration by measuring vapour pressure. Vapour pressure osmometers may miss certain volatiles that are in the solution, such as CO2, ammonia and alcohol.

Using a freezing point osmometer provides the most accurate and reliable results for many applications.

Melting point depression is not a colligative property. Mixtures of substances whose components are insoluble in each other in the liquid phase display a melting point depression and, instead of a fixed melting point, a melting range or interval.

The magnitude of the melting point depression is dependent on the composition of the mixture.

The depression of melting point is used to determine the purity and identity of compounds. EMLA (eutectic mixture of local anaesthetics) cream is a mixture of lidocaine and prilocaine. The melting point of the drugs combined is lower than that individually. The melting point is below room temperature (18°C).

Answer Statistics

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5%

2

68%

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4%

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4%

5

21%

Times answered: 224