Molality problems

Clinton Schmitz
10 Apr 201804:54
EducationalLearning
32 Likes 10 Comments

TLDRThis educational video script explains the concept of molality in chemistry, distinguishing it from molarity with the use of case-sensitive abbreviations (lowercase 'm' for molality and uppercase 'M' for molarity). It demonstrates how to calculate molality by dividing the moles of solute by the kilograms of solvent. Two examples are provided: converting 20 grams of sodium hydroxide and 18 grams of calcium chloride into moles and then calculating their respective molalities with water as the solvent, highlighting the process of converting grams to moles and milliliters to kilograms.

Takeaways
  • 🔍 Molality and molarity are different; molality is denoted by a lowercase 'M', while molarity is an uppercase 'M'.
  • 📚 The formula for molality is moles of solute divided by kilograms of solvent.
  • 🧪 To calculate molality, convert the mass of the solute to moles using its molar mass.
  • 📉 Convert the volume of the solvent in milliliters to kilograms by recognizing that 1 milliliter equals 1 gram, and there are 1000 grams in a kilogram.
  • 📝 For the first example, 20 grams of sodium hydroxide (NaOH) is converted to 0.500 moles using its molar mass of approximately 40 g/mol.
  • ⚖️ In the first example, 500 milliliters of water is converted to 0.500 kilograms for the molality calculation.
  • 🔢 The calculated molality for the first example is 1.000 moles per kilogram (M).
  • 📝 For the second example, 18 grams of calcium chloride (CaCl2) is converted to approximately 0.162 moles using its molar mass of approximately 111 g/mol.
  • ⚖️ In the second example, 450 milliliters of water is converted to 0.450 kilograms for the molality calculation.
  • 🔢 The calculated molality for the second example is 0.36 moles per kilogram (m).
  • 📚 The video encourages viewers to pause and attempt the second example before revealing the answer, promoting active learning.
Q & A

  • What is the difference between molality and molarity?

    -Molality is represented by a lowercase 'M' and is the ratio of moles of solute to kilograms of solvent. Molarity, represented by an uppercase 'M', is the ratio of moles of solute to liters of solution.

  • How is molality abbreviated in scientific notation?

    -Molality is abbreviated with a lowercase 'M'.

  • What is the formula for calculating molality?

    -The formula for calculating molality is moles of solute divided by kilograms of solvent.

  • How do you convert grams to moles in chemistry?

    -To convert grams to moles, divide the mass of the substance in grams by its molar mass.

  • What is the molar mass of sodium hydroxide (NaOH)?

    -The molar mass of sodium hydroxide (NaOH) is approximately 40 grams per mole, calculated by adding the atomic masses of sodium (23), oxygen (16), and hydrogen (1).

  • How do you calculate the molality of a solution with 20 grams of sodium hydroxide in 500 milliliters of water?

    -First, convert 20 grams of NaOH to moles (0.5 moles). Then, convert 500 milliliters of water to kilograms (0.5 kg). Finally, divide the moles of NaOH by the kilograms of water to get a molality of 1.00 mol/kg.

  • How many moles are in 18 grams of calcium chloride?

    -To find the moles, divide the mass of calcium chloride (18 grams) by its molar mass (111 grams/mole), resulting in approximately 0.16 moles.

  • What is the molality of a solution made by dissolving 18 grams of calcium chloride in 450 milliliters of water?

    -After converting 18 grams of calcium chloride to moles (0.16 moles) and 450 milliliters of water to kilograms (0.45 kg), divide the moles by the kilograms to get a molality of 0.36 mol/kg.

  • How do you convert milliliters of water to kilograms for the purpose of calculating molality?

    -Since one milliliter of water is approximately equal to one gram, you can convert milliliters to kilograms by dividing the volume in milliliters by 1000.

  • What is the significance of using lowercase 'm' for molality as opposed to uppercase 'M' for molarity?

    -The lowercase 'm' distinguishes molality from molarity, which is a different concentration measure. This notation helps avoid confusion between the two concepts.

  • Can you provide an example of how to calculate the molality of a solution with a different solute?

    -Certainly. For instance, if you have 50 grams of sucrose dissolved in 750 milliliters of water, first convert grams to moles using the molar mass of sucrose, then convert milliliters to kilograms, and finally divide the moles of solute by the kilograms of solvent to find the molality.

Outlines
00:00
📊 Understanding Molality vs Molarity

The video explains the difference between molality and molarity. Molality is abbreviated with a lowercase 'm' and is calculated as the moles of solute divided by the kilograms of solvent. This section emphasizes the importance of understanding these definitions to solve problems involving molality.

🧪 Solving a Molality Problem Step-by-Step

This segment demonstrates how to solve a molality problem. It involves converting 20 grams of sodium hydroxide into moles using its molar mass (NaOH: 23 for sodium, 16 for oxygen, and 1 for hydrogen, totaling 40). The video guides through dividing the moles of NaOH by the kilograms of solvent (500 milliliters converted to 0.500 kilograms), resulting in a molality of 1.00 moles per kilogram.

🔬 Practice Problem: Calculating Molality

A practice problem is introduced: calculating the molality of a solution with 18 grams of calcium chloride dissolved in 450 milliliters of water. The viewer is encouraged to pause and solve the problem independently before checking the answer in the video.

📝 Solution to Practice Problem

The video provides the solution to the practice problem. It converts 18 grams of calcium chloride into moles (CaCl2: 111 grams per mole, resulting in approximately 0.16 moles). The 450 milliliters of water is converted to 0.450 kilograms. The molality is calculated by dividing the moles of solute by the kilograms of solvent, resulting in a molality of 0.36 moles per kilogram.

🎓 Conclusion and Summary

The video concludes by summarizing the key points covered, ensuring the viewer understands how to calculate molality and differentiating it from molarity. The video ends with a reinforcement of the method and formula for solving molality problems.

Mindmap
Keywords
💡Molality
Molality is a measure of concentration in a solution, defined as the number of moles of solute per kilogram of solvent. It is different from molarity, which is moles of solute per liter of solution. In the video, molality is used to illustrate the concentration of the solute in the context of the 'mole alley' type problem, with the formula expressed as moles of solute divided by kilograms of solvent.
💡Molarity
Molarity is another measure of concentration, denoted by an uppercase 'M', which is the number of moles of solute per liter of solution. It is contrasted with molality in the video, where the lowercase 'm' is used to denote molality. The script emphasizes the difference in units and the importance of distinguishing between the two for solving problems.
💡Solute
A solute is the substance that is dissolved in a solvent to form a solution. In the context of the video, sodium hydroxide and calcium chloride serve as solutes in their respective solutions. The script explains how to convert the mass of the solute into moles, which is a crucial step in calculating molality.
💡Solvent
A solvent is the substance, usually a liquid, in which the solute is dissolved. Water is the solvent in the examples provided in the video. The script explains the conversion of the volume of water (in milliliters) to mass (in kilograms) for the calculation of molality.
💡Molar Mass
Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is used in the video to convert the mass of a solute into moles, which is essential for determining the molality of a solution.
💡Calcium Chloride
Calcium chloride is a chemical compound used as an example solute in the second problem of the video. The script demonstrates how to calculate the molality of a solution when calcium chloride is dissolved in water.
💡Sodium Hydroxide
Sodium hydroxide, also known as lye or caustic soda, is used as an example solute in the first problem of the video. The script walks through the process of converting grams of sodium hydroxide into moles and then calculating the molality of the solution.
💡Milliliters to Kilograms
The script explains the conversion of volume measurements (milliliters) to mass (kilograms) for water, which is necessary for calculating molality. Since one milliliter of water is approximately equal to one gram, the conversion involves moving the decimal point three places to the left.
💡Mole Alley Problem
The term 'mole alley problem' refers to a type of chemistry problem that involves calculating the molality of a solution. The video script provides a step-by-step guide on how to approach such problems, using specific examples to illustrate the process.
💡Formula
In the context of the video, the formula refers to the mathematical relationship used to calculate molality, which is moles of solute divided by kilograms of solvent. The script emphasizes the importance of writing down and applying the correct formula to solve the given problems.
Highlights

Introduction to the concept of molality and its distinction from molarity.

Molality is represented by a lowercase 'M', whereas molarity is represented by an uppercase 'M'.

Explanation of the formula for calculating molality: moles of solute divided by kilograms of solvent.

Demonstration of converting grams of sodium hydroxide to moles using molar mass.

Conversion of milliliters to kilograms for the solvent in the calculation of molality.

Calculation of molality for a solution with sodium hydroxide as the solute.

Illustration of how to simplify the conversion of milliliters of water to kilograms.

Result of the molality calculation for sodium hydroxide solution: 1.00 mol/kg.

Invitation for the audience to try a similar problem involving calcium chloride.

Step-by-step guide to convert grams of calcium chloride to moles for molality calculation.

Explanation of converting milliliters of water to kilograms for the solvent in the molality formula.

Final calculation of molality for a solution with calcium chloride as the solute: 0.36 mol/kg.

Emphasis on the importance of understanding the difference between molality and molarity for accurate calculations.

Highlighting the practical application of molality in chemistry and its relevance to real-world problems.

Encouragement for viewers to practice and apply the formula to enhance understanding of molality.

Conclusion of the tutorial with a summary of the steps and formulas for calculating molality.

Transcripts

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Molality CalculationChemistry TutorialSolute MolesSolvent KilogramsNaOH SolutionCalcium ChlorideChemical FormulasMolar MassMole ConversionScience EducationProblem Solving