For The Following Aqueous Solutions Calculate The Osmolality Chegg
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Osmolality is a measure of the total concentration of solutes in a solution, expressed in osmoles per kilogram of solvent. This calculator helps you determine the osmolality of aqueous solutions based on the types and amounts of solutes present.
What is Osmolality?
Osmolality refers to the total number of osmoles of solute particles per kilogram of solvent (usually water). It's an important concept in chemistry, biology, and medicine, particularly in understanding how solutions affect cell function and biological processes.
Unlike osmolarity, which measures osmoles per liter of solution, osmolality is independent of temperature and pressure changes, making it more useful for biological applications where temperature can vary.
Osmolality Formula
Osmolality Formula
Osmolality (m) can be calculated using the formula:
m = Σ (ni × vi) / msolvent
Where:
- ni = number of moles of solute i
- vi = van't Hoff factor for solute i
- msolvent = mass of solvent in kilograms
The van't Hoff factor (v) accounts for how many particles a solute dissociates into. For example, sodium chloride (NaCl) has a van't Hoff factor of 2 because it dissociates into two ions.
How to Calculate Osmolality
- Identify all solutes in the solution and their concentrations.
- Determine the van't Hoff factor for each solute.
- Calculate the number of moles of each solute.
- Sum the products of moles and van't Hoff factors for all solutes.
- Divide the total by the mass of the solvent in kilograms.
Important Notes
- Osmolality is typically expressed in milliosmoles per kilogram (mOsm/kg).
- For biological applications, osmolality is often measured in milliosmoles per kilogram (mOsm/kg).
- Common reference solutions include isotonic saline (0.9% NaCl) with an osmolality of approximately 300 mOsm/kg.
Example Calculation
Let's calculate the osmolality of a solution containing 0.9 g of NaCl in 1 kg of water.
- Molar mass of NaCl = 58.44 g/mol
- Moles of NaCl = 0.9 g / 58.44 g/mol ≈ 0.0154 mol
- Van't Hoff factor for NaCl = 2
- Total osmoles = 0.0154 mol × 2 = 0.0308 osmoles
- Osmolality = 0.0308 osmoles / 1 kg ≈ 30.8 mOsm/kg
This matches the osmolality of isotonic saline, which is commonly used in medical applications.
Osmolality vs. Osmolarity
While both terms measure solute concentration, they differ in their reference:
- Osmolality measures solute concentration relative to the mass of the solvent (kg).
- Osmolarity measures solute concentration relative to the volume of the solution (L).
Osmolality is more useful in biological systems where temperature and pressure can vary, as it remains relatively constant.
FAQ
What is the difference between osmolality and osmolarity?
Osmolality measures solute concentration per kilogram of solvent, while osmolarity measures solute concentration per liter of solution. Osmolality is more useful in biological contexts where temperature can vary.
What is the van't Hoff factor?
The van't Hoff factor is a number that accounts for how many particles a solute dissociates into. For example, NaCl has a van't Hoff factor of 2 because it dissociates into two ions.
What is the osmolality of isotonic saline?
The osmolality of isotonic saline (0.9% NaCl) is approximately 300 mOsm/kg.