Calculations Involving Osmolarity – PST05208 Pharmaceutics Theory and Compounding
NTA Level 5 • Semester 2 • PST05208 Calculations Involving Osmolarity Pharmaceutics Theory and Compounding • Source Session/Topic 18 Full source-text version: all educational wording from the extracted learning source is retained; only presenter/tutor metadata and web-layout noise are removed, while formatting is improved for readability. Session 18: Calculations Involving Osmolarity Total Session Time: 120 minutes + 6 hours of Practices Prerequisites • None Learning Tasks By the end of this session students are expected to be able to: • Give introduction to calculation involving Osmolarity • Concept on osmolarity vs. osmolality • Perform calculations Involving Osmolarity Resources Needed: • Flip charts, marker pens, and masking tape • Black/white board, chalk and whiteboard markers • LCD projector and compute SESSION OVERVIEW Activity/ Step Time Content Method 1 05 Minutes Presentation Introduction, Learning Tasks 2 40 Minutes Presentation Introduction to Calculation Involving Osmolarity 25 Minutes Presentation Concept on Osmolarity vs. Osmolality 40 Minutes Presentation Performing Calculation Involving Osmolarity 3 Demonstration 4 05 Minutes Presentation Key Points 5 05 Minutes Presentation Evaluation 125 SESSION CONTENTS STEP1: Presentation of Session Title and Learning Tasks (5 minutes) READ or ASK students to read the learning tasks and clarify ASK students if they have any questions before continuing STEP 2: Introduction to Calculation Involving Osmolarity (40 Minutes) Osmolarity Osmolarity is defined as the number of osmoles of solute per liter (L) of solution. It is expressed in terms of osmol/L or Osm/L. Osmolarity depends on the number of particles in a chemical solution, but not on the identity of those molecules or ions. Sample Osmolarity Calculations A 1 mol/L NaCl solution has an osmolarity of 2 osmol/L. A mole of NaCl dissociates fully in water to yield two moles of particles: Na+ ions and Cl- ions. Each mole of NaCl becomes two osmoles in solution. A 1 M solution of sodium sulfate, Na2SO4, dissociates into 2 sodium ions and 1 sulfate anion, so each mole of sodium sulfate becomes 3 osmoles in solution (3 Osm). To find the osmolarity of a 0.3% NaCl solution, you first calculate the molarity of the salt solution and then convert the molarity to osmolarity. Convert percent to molarity: 0.03 % = 3 grams 100 ml = 3 grams / 0.1 L = 30 g/L molarity NaCl = moles / liter = (30 g/L) x (1 mol / molecular weight of NaCl) Look up the atomic weights of Na and Cl on the periodic table and add the together to get the molecular weight. Na is 22.99 g and Cl is 35.45 g, so the molecular weight of NaCl is 22.99 + 35.45, which is 58.44 grams per mole. Plugging this in: Molarity of the 3% salt solution = (30 g/L) / (58.44 g/mol) molarity = 0.51 M You know there are 2 osmoles of NaCl per mole, so: Osmolarity of 3% NaCl = molarity x 2 Osmolarity = 0.51 x 2 Osmolarity = 1.03 Osm 126 STEP 3: Concept on Osmolarity vs. Osmolality (25 Minutes) • Osmolality and osmolarity are units of measurement. Osmolality is the number of osmoles of solute in a kilogram of solvent, while osmolarity is the number of osmoles of solute in a litre of solution. Osmolarity is the concentration of an osmotic solution. • Osmolality is convenient to use because the amount of solvent remains constant, regardless of changes in temperature and pressure. • While osmolarity is easy to calculate, it's less difficult to determine because the volume of solution changes according to temperature and pressure. Osmolarity is most commonly used when all measurements are made at a constant temperature and pressure. • If osmolality is the number of osmoles of solute in a kilogram of solvent, then osmolarity is the number of osmoles of solute in a litre of solution. • Osmolarity deals with the concentration of an osmotic solution, while osmolality deals with the concentration of particles in a fluid. • It is easier to determine the osmolality than the osmolarity. Osmolarity is expressed as Osm/L, and osmolality is expressed as Osm/Kg. • Osmolality is used to determine medical conditions like diabetes, shock and dehydration, while osmolarity is used for the detection of the concentration of dissolved particles in urine.Osmolality is the commonly used method of measurement in Osmometry. • When the concentration of solutes is very low, the osmolality and osmolarity are similar. 127 STEP 3: Performing Calculations Involving Osmolarity (40 Minutes) Activity: Small Group Discussion ( 20 minutes) DIVIDE students in small manageable groups ASK students to discuss in groups on the following questions • How many mOsmol are represented in a liter of a 0.9% sodium chloride solution REFER Students to Pharmaceutical Calculation. 13th Edition by HOWARD C. ANSEL: Chapter 11, for reference ALLOW students to discuss for 20 minutes ALLOW each groups to present for 5 minutes CLARIFY and SUMMARIZE by using the contents below Formula weight of NaCl = 58.5 1 mmol of NaCl (= 58.5mg) = 2 mOsmol 900mg of NaCl per 100mL = 9000mg NaCl per liter Then by using proportion: 58.5mg = 2mOsmol x = 9000 x 2 9000mg x mOsmol 58.5 X = 307.7 = 308mOsmol/L STEP 4: Key Points (5 Minutes) • Milliosmoles (mOsmol) is an expression of the osmotic activity of 1 Millimole • A distinction also should be made between the terms osmolarity and osmolality. Whereas Osmolarity is the milliosmoles of solute per liter of solution while osmolality is the milliosmoles of solute per kilogram of solvent. For dilute aqueous solutions, osmolarity and osmolality are nearly identical. • For more concentrated solutions, however, the two values may be quite dissimilar. The Pharmaceutical personnel should pay particular attention to a product‟s label statement regarding molarity versus molality. 128 STEP 5: Evaluation (5 Minutes) • What is Milliosmoles? • What are the difference between osmolarity and osmolality? STEP 6: Take Home Assignment (15 minutes) Activity: Take home Assignment (15 minutes) ASK each individual student to do the following assignment • A solution contains 156mg of k+ ions per 100 mL. How