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What is Molality (chemistry)?
Grade Level:
Class 7
Space Technology, EVs, Climate Change, Biotechnology, HealthTech, Robotics, Chemistry, Physics
Definition
What is it?
Molality tells us how concentrated a solution is by measuring the amount of a substance (solute) dissolved in a certain amount of another substance (solvent). It specifically focuses on the mass of the solvent, not the total volume of the solution.
Simple Example
Quick Example
Imagine you are making chai. Molality would be like measuring how much sugar (solute) you add for every kilogram of milk (solvent), instead of measuring it for the entire cup of chai. If you add 10 grams of sugar to 1 kg of milk, that's a specific molality.
Worked Example
Step-by-Step
Let's find the molality of a solution where 5.85 grams of common salt (NaCl) are dissolved in 500 grams of water.
Step 1: Find the molar mass of NaCl. (Na = 23 g/mol, Cl = 35.5 g/mol). Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol.
---Step 2: Calculate the number of moles of NaCl. Moles = Given mass / Molar mass. Moles of NaCl = 5.85 g / 58.5 g/mol = 0.1 mol.
---Step 3: Convert the mass of solvent (water) from grams to kilograms. Mass of water = 500 g = 0.5 kg.
---Step 4: Use the formula for molality: Molality (m) = Moles of solute / Mass of solvent (in kg).
---Step 5: Substitute the values. Molality = 0.1 mol / 0.5 kg = 0.2 mol/kg.
Answer: The molality of the solution is 0.2 m (or 0.2 mol/kg).
Why It Matters
Understanding molality is super important in fields like Biotechnology for preparing precise chemical mixtures, or in HealthTech to create medicines with exact concentrations. Scientists and engineers use it to design new materials and understand how chemicals react, even for things like better batteries in EVs!
Common Mistakes
MISTAKE: Using the volume of the solution instead of the mass of the solvent. | CORRECTION: Molality always uses the mass of the solvent in kilograms, not the total volume of the solution.
MISTAKE: Forgetting to convert the mass of the solvent from grams to kilograms. | CORRECTION: Always convert the mass of the solvent to kilograms before using it in the molality formula.
MISTAKE: Confusing molality with molarity. | CORRECTION: Molality uses mass of solvent (kg), while molarity uses volume of solution (litres). They are different!
Practice Questions
Try It Yourself
QUESTION: What is the molality of a solution containing 11.7 grams of NaCl in 100 grams of water? (Molar mass of NaCl = 58.5 g/mol) | ANSWER: 2 mol/kg
QUESTION: A solution contains 90 grams of glucose (C6H12O6) dissolved in 250 grams of water. Calculate the molality of the solution. (Molar mass of glucose = 180 g/mol) | ANSWER: 2 mol/kg
QUESTION: If 0.5 moles of urea are dissolved in 750 grams of solvent, what is the molality? If the mass of the solvent was doubled, how would the molality change? | ANSWER: 0.67 mol/kg. If the solvent mass doubles, molality would halve (become 0.335 mol/kg).
MCQ
Quick Quiz
Which of the following units is used for molality?
moles per litre
grams per litre
moles per kilogram
grams per kilogram
The Correct Answer Is:
C
Molality is defined as moles of solute per kilogram of solvent. Therefore, 'moles per kilogram' is the correct unit. The other options refer to different concentration units or are incorrect.
Real World Connection
In the Real World
In India, chemists in pharmaceutical companies use molality to precisely formulate medicines, ensuring each dose has the correct amount of active ingredient. For example, if you're making a cough syrup, knowing the molality helps ensure the active medicine concentration is just right, regardless of temperature changes that might affect volume.
Key Vocabulary
Key Terms
SOLUTE: The substance that gets dissolved (e.g., sugar in chai) | SOLVENT: The substance that does the dissolving (e.g., milk in chai) | SOLUTION: The mixture formed when solute dissolves in solvent | MOLES: A unit for measuring the amount of a substance (like a 'dozen' but for atoms/molecules) | MOLAR MASS: The mass of one mole of a substance
What's Next
What to Learn Next
Great job understanding molality! Next, you should explore 'Molarity'. It's another way to express concentration, but it uses the volume of the solution instead of the mass of the solvent, which is important for different situations.
