Chemistry · Some Basic Concepts Of Chemistry · NEET
Take the NCERT example: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g). It tells you three things at once. (1) Molecule ratio: 1 molecule of methane reacts with 2 molecules of oxygen to give 1 CO2 and 2 water. (2) Mole ratio: 1 mole CH4 : 2 moles O2 : 1 mole CO2 : 2 moles H2O. (3) Mass balance: total mass on the left equals total mass on the right (16 + 64 = 80 g = 44 + 36). The (g) letters also tell you the physical state (gas here).
Both, and that is the key idea. The coefficient 2 in front of O2 means 2 molecules of O2 at the tiny scale, OR 2 moles of O2 at the lab scale. Because 1 mole always contains the same number of particles (6.022 x 10^23), a ratio of molecules is the same as a ratio of moles. So 1:2:1:2 works whether you count single molecules or whole moles. This is why NEET problems jump straight from the equation to moles.
No, not directly. Coefficients give you the MOLE ratio, not the mass ratio. To get mass, multiply each coefficient by that substance's molar mass. For CH4 + 2O2 -> CO2 + 2H2O: mass of CH4 = 1 x 16 = 16 g, mass of O2 = 2 x 32 = 64 g. So the mass ratio of CH4 to O2 is 16:64 = 1:4, which is NOT the same as the mole ratio 1:2. This mix-up is a very common exam mistake.
Because of the Law of Conservation of Mass: atoms are not created or destroyed in a reaction, only rearranged. A balanced equation has the same number of atoms of every element on both sides, so the total mass must match. In CH4 + 2O2 -> CO2 + 2H2O, both sides have 1 C, 4 H, and 4 O atoms. That is exactly what 'balanced' means.
A balanced equation does NOT tell you: how fast the reaction happens (rate), whether it needs heat or a catalyst unless written above the arrow, how much energy is released or absorbed unless it is a thermochemical equation, or whether the reaction actually goes fully forward. It only gives the ratio of amounts (moles/molecules) and confirms mass balance. Do not read speed or 'ease' from it.
Use the mole ratio as a bridge. Example (NEET 2019): For N2 + 3H2 -> 2NH3, how much H2 makes 20 mol NH3? Read the ratio: 2 mol NH3 needs 3 mol H2. So H2 = (3/2) x 20 = 30 mol. Always: convert given data to moles, multiply by the coefficient ratio, then convert back to whatever the question asks (mass, volume, or particles).
The number of moles of hydrogen molecules required to produce 20 moles of ammonia through the Haber process is
For the Haber process N2 + 3H2 -> 2NH3, which information can be read directly from the balanced equation? (concept check on same reaction) The mole ratio of N2 : H2 : NH3 is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Not directly. It gives the mole ratio. To get mass, multiply each coefficient by that substance's molar mass. Only then can you compare masses.
No. Coefficients are the big numbers in front (they scale the whole formula and give the mole ratio). Subscripts are the small numbers inside a formula (like the 4 in CH4) and you must never change them to balance an equation.
No. It only tells the ratio of amounts and confirms mass balance. Speed (rate), catalyst, and energy are separate information and are not shown by the coefficients alone.
Because 1 mole always has the same number of particles (6.022 x 10^23). So a ratio of molecules equals the same ratio of moles. The coefficient 2 means 2 molecules or 2 moles, whichever scale you use.
Write and balance the equation, then convert the given amount to moles. Use the coefficient ratio to find moles of the target substance, then convert to the unit the question wants.
