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1. Matter and Measurements4h 31m
2. Atoms and the Periodic Table5h 23m
3. Ionic Compounds2h 18m
4. Molecular Compounds2h 18m
5. Classification & Balancing of Chemical Reactions3h 17m
6. Chemical Reactions & Quantities2h 27m
7. Energy, Rate and Equilibrium3h 46m
8. Gases, Liquids and Solids3h 25m
9. Solutions4h 10m
10. Acids and Bases3h 29m
11. Nuclear Chemistry56m
BONUS: Lab Techniques and Procedures1h 38m
BONUS: Mathematical Operations and Functions47m
12. Introduction to Organic Chemistry1h 34m
13. Alkenes, Alkynes, and Aromatic Compounds2h 12m
14. Compounds with Oxygen or Sulfur1h 6m
15. Aldehydes and Ketones1h 1m
16. Carboxylic Acids and Their Derivatives1h 11m
17. Amines39m
18. Amino Acids and Proteins1h 51m
19. Enzymes1h 37m
20. Carbohydrates1h 41m
21. The Generation of Biochemical Energy2h 8m
22. Carbohydrate Metabolism2h 22m
23. Lipids2h 26m
24. Lipid Metabolism1h 45m
25. Protein and Amino Acid Metabolism1h 37m
26. Nucleic Acids and Protein Synthesis2h 54m

9. Solutions

Molality

9. Solutions

Molality: Videos & Practice Problems

Video Lessons Practice Worksheet

Topic summary

Ionic molality, or osmolality, measures the concentration of dissolved ions in a solution. For instance, a 0.30 molal sodium chloride solution dissociates into two ions: sodium and chloride. The osmolality is calculated as the product of the number of ions and the molality, resulting in an osmolality of 0.60. Understanding osmolality is crucial for applications in chemistry and biology, particularly in processes like osmosis and cellular transport.

Molality (m) represents the amount of solute dissolved per kilogram of solvent.

Understanding Molality

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Molality

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Molality Video Summary

Molality, represented by the lowercase letter m, is defined as the number of moles of solute per kilogram of solvent. This concept is crucial in understanding the composition of solutions, where the solute is the smaller component and the solvent is the larger component. In contrast, molarity, denoted by the capital letter M, measures the moles of solute per liter of solution. Due to their similarities, it is common to encounter problems that require converting between molality and molarity.

To illustrate molality, consider a solution containing 0.30 moles of sodium chloride (NaCl). This indicates that there are 0.30 moles of NaCl dissolved in 1 kilogram of solvent, typically water. Similarly, if we have a 0.25 molar solution of glucose (C₆H₁₂O₆), this means there are 0.25 moles of glucose in 1 liter of solution, which also implies that the solvent is water, equating to approximately 1 kilogram.

Understanding these definitions is essential, especially when converting molality to other concentration measures such as molarity, mole fraction, or mass percent. Remember, when given the molality of a solution, it directly translates to the number of moles of solute per kilogram of solvent, a fundamental concept in solution chemistry.

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Osmolality

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Osmolality Video Summary

Ionic molality, also known as osmolality, refers to the concentration of dissolved ions in a solution. To calculate the osmolality of a solution, it is essential to consider the dissociation of the solute into its constituent ions. For instance, when sodium chloride (NaCl) is dissolved in water, it dissociates into two ions: sodium ions (Na⁺) and chloride ions (Cl^-). This means that for every mole of sodium chloride, there are two moles of ions produced.

To find the osmolality, you can use the formula:

Osmolality = Number of ions × Molality

In the case of a 0.30 molal solution of sodium chloride, the calculation would be:

Osmolality = 2 × 0.30 molal = 0.60 osmolal

This result indicates that the ionic molality or osmolality of the solution is 0.60 osmolal. It is crucial to remember that when calculating osmolality, one must account for the total number of ions produced from the dissociation of the solute in the solution. To reinforce your understanding, try the example provided at the bottom of the page and compare your answer with the calculated result.

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Calculate Molality Example

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Molality Calculations

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Molality Calculations Example 1

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Molality Calculations Example 2

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Problem

What is the mass percent of NH₃ of a 1.25 m aqueous solution of NH₃?

1.11%

3.53%

2.08%

87.4%

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Here’s what students ask on this topic:

What is the difference between molality and osmolality?

Molality (m) is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. Osmolality, on the other hand, measures the concentration of all dissolved particles (ions, molecules) in a solution. It is calculated by multiplying the molality of the compound by the number of particles into which the compound dissociates. For example, a 0.30 molal sodium chloride (NaCl) solution dissociates into two ions (Na⁺ and Cl^-), resulting in an osmolality of 0.60 osmol/kg.

How do you calculate the osmolality of a solution?

To calculate the osmolality of a solution, you need to know the molality of the solute and the number of particles it dissociates into. The formula is:

$Osmolality = n \times m$

where $n$ is the number of particles the solute dissociates into, and $m$ is the molality of the solution. For example, for a 0.30 molal NaCl solution, which dissociates into 2 ions, the osmolality is:

$2 \times 0.30 = 0.60$ osmol/kg.

Why is osmolality important in biological systems?

Osmolality is crucial in biological systems because it affects the movement of water across cell membranes through osmosis. Cells maintain a specific osmolality to ensure proper hydration, nutrient uptake, and waste removal. An imbalance in osmolality can lead to cell shrinkage or swelling, disrupting cellular function and potentially causing damage. For example, in medical settings, intravenous fluids are often adjusted to match the osmolality of blood to prevent adverse effects on cells.

What is the osmolality of a 0.50 molal solution of potassium sulfate (K₂SO₄)?

Potassium sulfate (K₂SO₄) dissociates into three ions: 2 potassium ions (K⁺) and 1 sulfate ion (SO₄^2-). To calculate the osmolality, use the formula:

$Osmolality = 3 \times 0.50 = 1.50$ osmol/kg.

So, the osmolality of a 0.50 molal K₂SO₄ solution is 1.50 osmol/kg.

How does temperature affect molality and osmolality?

Molality is independent of temperature because it is based on the mass of the solvent, which does not change with temperature. Osmolality, similarly, is not directly affected by temperature since it depends on the number of dissolved particles per kilogram of solvent. However, temperature can influence the dissociation of solutes and the solubility of compounds, indirectly affecting the osmolality if the number of dissociated particles changes.