Chemistry · Some Basic Concepts Of Chemistry · NEET
It is the whole number written BEFORE a chemical formula in a balanced equation. In CH4 + 2O2 -> CO2 + 2H2O, the '2' in front of O2 and H2O are stoichiometric coefficients. NCERT says they represent the number of molecules — and also the number of moles — taking part in or formed in the reaction. So this one number links the microscopic (molecules) world to the lab (moles) world.
This is the #1 mix-up. A COEFFICIENT is the big number in FRONT of the formula and it multiplies the WHOLE formula (it counts how many units). A SUBSCRIPT is the small low number INSIDE the formula and it counts atoms of one element in one molecule. Example: 2H2O means 2 whole water molecules; the subscript 2 in H2O means each water has 2 hydrogen atoms. So 2H2O = 4 H atoms and 2 O atoms in total. You can change a coefficient to balance an equation, but you must NEVER change a subscript — that would make a different substance.
It multiplies the WHOLE formula, every atom inside it. In 3H2SO4, the 3 multiplies everything: you get 3×2 = 6 H atoms, 3×1 = 3 S atoms, and 3×4 = 12 O atoms. A common wrong idea is that 3 only affects the H. It affects all atoms equally.
Read the coefficients as a ratio. In N2 + 3H2 -> 2NH3, the ratio is 1 : 3 : 2. This means 1 mole N2 reacts with 3 moles H2 to make 2 moles NH3. It also scales: 10 moles N2 need 30 moles H2 and give 20 moles NH3. The coefficients are the bridge you use for every mole-to-mole calculation in NEET.
If no number is shown in front of a formula, the coefficient is 1 (we never write the 1). In CH4 + 2O2 -> CO2 + 2H2O, the coefficient of CH4 and of CO2 is 1 each. So 1 mole CH4 gives 1 mole CO2. Missing number always means one.
Coefficients directly give the MOLE ratio and the MOLECULE ratio, not the mass ratio. To get mass you multiply each substance's moles by its molar mass. But total mass is still conserved (law of conservation of mass) — the same number of each atom appears on both sides, just grouped differently.
Yes, sometimes a balanced equation is written with a fraction, like H2 + 1/2 O2 -> H2O. This is correct and often used in thermochemistry. To make all coefficients whole numbers you can multiply the whole equation by 2: 2H2 + O2 -> 2H2O. Both forms describe the same reaction; the ratio stays the same.
The number of moles of hydrogen molecules required to produce 20 moles of ammonia through the Haber process is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the number in front of a formula in a balanced equation that tells how many moles (and molecules) of that substance take part in the reaction.
Changing a coefficient only changes HOW MANY units you have. Changing a subscript changes the substance itself (H2O into H2O2 is a different compound), so it is not allowed.
Usually yes, and the smallest whole-number ratio is preferred. Fractions are allowed (common in thermochemistry) and can be cleared by multiplying the whole equation by a number.
Almost every mole, mass, volume, and limiting-reagent numerical starts by reading the coefficient ratio from the balanced equation. Getting the ratio wrong makes the whole calculation wrong.
