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🧪 NEET Chemistry❓ Question

Molarity, molality and mole fraction: which depends on temperature and how do I not mix up the denominators?

Amma says 'recheck the basics' every time I get a calculation wrong, and she's right.

Kavya NaikDropper Sample

4 Oct 2026 · 1 min read

Repeater year, and the same silly mistakes in concentration terms that I made last year. I keep mixing the denominators, and I'd like to finally fix that.

What I have:

M=moles of solutevolume of solution (L),m=moles of solutemass of solvent (kg),xA=nAnA+nBM = \frac{\text{moles of solute}}{\text{volume of solution (L)}}, \quad m = \frac{\text{moles of solute}}{\text{mass of solvent (kg)}}, \quad x_A = \frac{n_A}{n_A+n_B}

My doubts:

  1. Which of these change if I heat the solution? I remember something about 'volume-based terms' but not why.
  2. A question from a mock: 18 g of glucose (M=180M = 180) is dissolved in 100 g of water. Find the molality and the mole fraction of glucose. I got molality as 0.1/0.118=0.850.1/0.118 = 0.85 and I don't know where I went wrong.
  3. Is 1 M the same as 1 m? A friend said that in dilute aqueous solutions they're nearly equal. Is that true or a myth?
  4. What does 'normality' mean now? Do I need it for NEET?

Please correct my formula list also if something is off.

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6 answers & replies

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  • [+]Arjun Nair12thSample· 16d ago

    Temperature: volume changes with temperature (liquids expand), mass does not. So anything with volume in the denominator, molarity (and normality), changes with T. Molality, mole fraction, and mass percent use only mass or moles, so they don't change with temperature. That's why molality is preferred in physical chemistry (e.g., colligative properties) where you heat or cool a solution.

  • [+]Dev Agarwal12thSample· 16d ago

    In molality you divide by the mass of solution. Total mass is 118 g, so 0.1 mol / 0.118 kg = 0.85 m. So you are right Kavya, and the bank's answer key must have used the same. The word 'solvent' in the definition just means the whole liquid.

    • [+]Neel Chatterjee12thSample· 16d ago

      Dev, no, molality is moles of solute per kg of solvent, and 'solvent' means only the water (100 g), not the glucose plus water. Kavya's mistake is exactly this one.

      m=18/1800.100 kg=0.10.1=1.0 mm = \frac{18/180}{0.100\ \text{kg}} = \frac{0.1}{0.1} = 1.0\ m

      Mole fraction: moles of water =100/18=5.56= 100/18 = 5.56.

      xglucose=0.10.1+5.56=0.15.66≈0.0177x_{glucose} = \frac{0.1}{0.1+5.56} = \frac{0.1}{5.66} \approx 0.0177

      So molality 1.0 m and x≈0.0177x \approx 0.0177.

      • [+]Kavya NaikDropperSample· 15d agoAuthor

        ayyo, I divided by the total mass 😭 'solvent underline' is now written on my hand. Thank you everyone!

  • [+]Aarav Kulkarni11thSample· 15d ago

    My checklist for these, since I love lists:

    1. Write what is in the numerator (always moles of solute, except mole fraction and mass %).
    2. Write the denominator and its unit: solution litres (M), solvent kg (m), total moles (x).
    3. Write the formula for the word 'solution', 'solvent' underlined.
    4. Ask if it asks for T-dependent quantities. It sounds silly but it forces me not to hurry.
  • [+]Nisha Verma12thSample· 15d ago

    For (3): for dilute aqueous solutions, 1 L of solution has mass close to 1 kg, almost all of it water, so M and m are nearly equal. In concentrated solutions or in solvents with density very different from 1 g/mL, they differ a lot. Not a myth, but it's an approximation, not an identity. Normality (equivalents per litre) is not in the current NCERT chapter, so you can ignore it for NEET unless some old book brings it in. Dilution formula to keep handy: M1V1=M2V2M_1V_1 = M_2V_2.

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