Some Basic Concepts of Chemistry
Start with one particle and keep zooming out ×100. A classroom, a city, all of India, every star in the universe — the real-world comparisons run out before you ever reach one mole.
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Avogadro's number, denoted Nₐ, is 6.022 × 10²³ per mole. It is the number of carbon-12 atoms in exactly 12 grams of pure carbon-12, and by definition the number of entities in one mole of any substance. NEET expects this value memorised to four significant figures.
Almost every stoichiometry, concentration, and gas-law problem in NEET starts by converting grams or litres into moles. The mole concept links the atomic-scale world (atoms and molecules) to the lab-scale world (grams and millilitres), which is why it underlies the entire Class 11 chemistry syllabus.
0.5 mole of CO₂ contains 0.5 × 6.022 × 10²³ = 3.011 × 10²³ molecules. Each CO₂ molecule has 2 oxygen atoms, so the total oxygen atom count is 2 × 3.011 × 10²³ = 6.022 × 10²³. That is exactly one mole of oxygen atoms.
At Standard Temperature and Pressure (273.15 K and 1 bar in the modern definition), one mole of any ideal gas occupies 22.7 litres. The older NCERT value of 22.4 L is based on 1 atm pressure and is still widely accepted in NEET problems.
6.022 × 10²³ is so large that even if every particle were rendered as a single pixel, no screen on Earth has enough pixels to display them all. The lab shows the cube going dark because particles overlap by the billions per pixel — visual proof of just how massive Avogadro's number is.
How many oxygen atoms are present in 0.5 mole of CO₂?