From mass per cent to a molecular formula
"From mass per cent to a molecular formula" is a free, interactive Class 11 Chemistry simulation for NEET and JEE, built on the NCERT section "Percentage composition, empirical and molecular formula" in Some Basic Concepts of Chemistry. A 100 g sample is split by mass percent into element piles, each converted to moles, divided by the smallest and scaled to whole numbers, then multiplied by n from the molar mass. NEET has asked about this idea 2 times in 10 years of papers. JEE Main asked 45 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Percentage composition, empirical and molecular formula in Some Basic Concepts of Chemistry. Change the values and watch what happens.
What it shows
NCERT § "Percentage Composition"
A 100 g sample is split by mass percent into element piles, each converted to moles, divided by the smallest and scaled to whole numbers, then multiplied by n from the molar mass.
Controls
4 to try
- Slider%C 0–90
- Slider%H 0–20 (O is the remainder)
- Segmentedmultiply ratio by ×1 / ×2 / ×3
- Slidermolar mass 30–300 g/mol
Try this
Given 40.0% C, 6.7% H and molar mass 180 g/mol, reach the molecular formula C6H12O6: set the multiplier and n so the readout atoms are whole numbers and the formula mass equals 180 ± 1.
Common mistake
Stopping at the empirical formula when molar mass (or vapour density) is given.
In NEET
10 years of papers
Asked 2 times in 10 years of NEET.
See the past questions: Mole Concept, Some Basic Concepts of Chemistry (chapter)
In JEE Main
42 shifts analysed
45 questions from Some Basic Concepts of Chemistry across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT § "Percentage Composition"
- Mass percent of an element = (its mass in one mole of the compound / molar mass of the compound) × 100.
- The empirical formula gives the simplest whole-number ratio of atoms; the molecular formula gives the actual number of each kind of atom in a molecule.
- Route from percentages to empirical formula: assume 100 g, convert each mass to moles, divide all by the smallest mole value, then multiply by a small integer if needed to reach whole numbers.
- Molecular formula = n × (empirical formula), where n = molar mass / empirical formula mass, and n is a whole number.
- Glucose (C₆H₁₂O₆) and ethanoic acid (C₂H₄O₂) both have the empirical formula CH₂O, so the empirical formula alone cannot identify a compound.
- Background, not stated in this NCERT chapter: if a problem gives vapour density relative to hydrogen, molar mass is about twice that value, a result that follows from Avogadro's law.
Questions
5 answers
- What does the "From mass per cent to a molecular formula" simulation show?
- A 100 g sample is split by mass percent into element piles, each converted to moles, divided by the smallest and scaled to whole numbers, then multiplied by n from the molar mass.
- What mistake does the "From mass per cent to a molecular formula" simulation fix?
- A common mistake it clears up: stopping at the empirical formula when molar mass (or vapour density) is given.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 2 times in 10 years of papers.
- Does JEE Main ask questions from Some Basic Concepts of Chemistry?
- Yes. JEE Main asked 45 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Percentage composition, empirical and molecular formula" in Some Basic Concepts of Chemistry (Class 11 Chemistry).
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