BIO 2101 all 9-07-2023

Osmolarity and Molarity Review

Understanding Concentrations

• Different types of concentration measures include:

  • Percent Concentration: Represents the amount of solute in a certain volume of solution, typically expressed as grams of solute per 100 mL of solution. It's commonly used in biology labs, but it's essential to convert it to molarity or osmolarity for biochemical calculations.

  • Molarity (M): Defined as the number of moles of solute per liter of solution. Molarity provides a direct correlation between solute quantity and its concentration in solution, making it crucial for stoichiometric calculations.

  • Osmolarity (Osm): Refers to the total concentration of solute particles in a solution, expressed in osmoles per liter. It considers both the number of solute particles and their dissociation behavior in solution, which is vital for physiological processes.

Example Problem: Calculating Percent from Glucose Solution

Given a 1.5 Molar Glucose solution:

• Molecular Weight of Glucose: 180 g/mol.

• Calculation for percent:

  • 1.5 moles of glucose equals 270 grams per liter (calculated as 1.5 moles x 180 g/mol = 270 g).

  • Convert grams to percent: To convert grams per liter to percent concentration, divide by 10. Therefore, 270 g/L = 27% concentration.

Molarity Calculation Steps

Finding Molarity:

• Solid concentration calculations often round the volume for convenience (e.g., using 100 mL or 250 mL as standard volumes).

• Example: Consider calcium bromide (CaBr2) with a molar mass of 40.08 g/mol for Calcium + 2 x 79.9 g/mol for Bromine results in a total molecular weight of 199.88 g/mol.

• If a sample contains 0.214g in 250mL of solution, convert it to molarity through scaling:

  • Scale to 1L: 0.214g x 4 = 0.856g in 1L.

  • Calculate moles: Moles = 0.856g / 199.88 g/mol = approximately 0.00429 mol/L.

Calculating Osmolarity

Osmolarity Factor:

• The osmolarity factor is crucial for understanding how solutes behave in solution. Differentiate between ionic and covalent compounds:

  • Ionic Compounds: Dissociate into their respective ions in solution. For example, Calcium Chloride (CaCl2) produces three ions (1 Ca2+ and 2 Cl-), thus having an osmolarity factor of 3.

  • Covalent Compounds: These remain as single molecules in solution (like glucose) and have an osmolarity factor of 1.

• Formula: Osmolarity = Molarity x Osmolarity Factor.

  • Example: A sodium chloride solution with a molarity of 2 means osmolarity is 2 x 2 (since NaCl disassociates into two ions) = 4 osmoles. Similarly, for magnesium bromide, the osmolarity is 2 Molarity x 3 (from 3 dissociated ions) = 6 osmoles; for glucose, it remains 1 osmole.

Clinical Application

Significance in Medicine:

• Understanding osmolarity is vital in medicine because it helps ensure that isotonic solutions are used in medical applications, which typically range between 270-310 mOsm/L for human cells.

• Common examples of isotonic solutions used in clinical settings include:

  • 0.85% Sodium Chloride: Also known as physiological saline.

  • 5% Dextrose: Used for IV therapy to provide energy.

• Quantitative calculations for medical solutions must consider mOsm values: A coherent understanding of osmolarity is critical for proper IV solutions and fluid therapy.

Important Conversion Tips

Use of Calculators:

• Always calculate to completion with the calculator to avoid rounding errors in intermediate steps.

• Pay careful attention to significant figures, especially in scientific calculations, as these can greatly affect the outcome.

Osmosis Principles

Osmosis Overview:

• Osmosis is the principle that describes how water moves concerning solute concentration gradients. Water naturally flows from areas of lower solute concentration to areas of higher solute concentration in an attempt to equalize concentrations on either side of a semipermeable membrane.

• One practical example is using salt (sodium chloride) in the salting of meat; the salt creates a hypertonic environment, effectively drawing water out of the meat and preserving it through dehydration.

Theoretical Application in Problems

• Interpreting Percent Solutions: Use calculations from percent solutions, such as a 3.56% Calcium Chloride solution, to derive osmolarity and compare it with another solution like magnesium bromide.

• Osmosis Predictions: Predict the directional flow of osmosis based on calculated osmolarities to comprehend how solutions behave when separated by membranes with different solute concentrations.

Molarity vs. Molality

Understanding Differences:

• Molarity (M): Represents the moles of solute per liter of solution, sensitive to temperature changes because it depends on volume.

• Molality (m): Represents moles of solute per kilogram of solvent (water). It's temperature-independent as it depends on mass, not volume, making it more reliable in varying temperatures.

• In normal physiological contexts, differences between molarity and molality are often negligible since dilute solutions predominantly use water as the solvent.

Conclusion and Recommendations

Preparation for Upcoming Exam:

• Ensure a thorough review and understanding of equations for calculating osmolarity and molarity to master exam material.

• Familiarize yourself with the periodic table and the molecular weights of common solutes as they are crucial for these calculations.

• Utilize flashcards or detailed notes to track and quickly reference essential information before the exam, ensuring readiness and confidence.