Chemistry · Some Basic Concepts Of Chemistry · NEET
Follow 3 fixed steps every time. Step 1: change the given amount to moles (moles = mass / molar mass; for gases at STP you can use volume / 22.4 L). Step 2: multiply by the mole ratio, which is (coefficient of what you want) divided by (coefficient of what you were given), taken straight from the balanced equation. Step 3: change your answer moles back to what the question asks (multiply by molar mass for grams, or by 22.4 L for gas volume at STP). NEET questions almost never skip this pattern, so if you memorise these 3 steps you can solve most of them.
The mole ratio is the ratio of the big numbers (coefficients) written in front of the formulas in a balanced equation. For N2 + 3H2 -> 2NH3, the ratio of H2 to NH3 is 3 to 2. This means every 2 moles of NH3 need 3 moles of H2. You always read the ratio directly from the balanced equation, not from the masses. This is the heart of stoichiometry, so read the coefficients carefully.
Because of the Law of Conservation of Mass, atoms cannot appear or disappear in a reaction. A balanced equation has the same number of each atom on both sides, so the coefficients correctly show how many moles react. If the equation is not balanced, your mole ratio is wrong and every answer after that is wrong. So the very first thing to do is check that the equation is balanced.
Always use MOLES for the ratio, never grams. The coefficients in an equation count particles (moles), not mass. For example, 2 mol H2 does not weigh the same as 2 mol NH3, but the equation still relates them 3:2 by moles. So convert to moles first, apply the ratio in moles, then convert back to grams only at the end.
This is called a mass-to-mass problem. Grams of A -> divide by molar mass of A -> moles of A -> multiply by mole ratio (B/A) -> moles of B -> multiply by molar mass of B -> grams of B. It is just the 3 steps with a mass unit at both ends. NEET loves this, for example finding grams of CO2 made when butane burns.
At STP (old value), 1 mole of any gas takes 22.4 L, so moles = volume in L / 22.4. NCERT now also uses 22.7 L at the new STP (273.15 K, 1 bar), so read what the question gives. Convert the gas volume to moles, use the mole ratio, then convert back. For gases reacting with gases you can even use volume ratios directly (Gay-Lussac's law) without moles.
The number of moles of hydrogen molecules required to produce 20 moles of ammonia through the Haber process is
The amount of carbon dioxide evolved upon complete combustion of 116 g of n-butane is (Atomic masses: H = 1, C = 12, O = 16)
What mass of pure CaCO3 is required to neutralise 50 mL of 0.5 M HCl according to CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + CO2(g) + 2H2O(l)?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No, it is one of the most scoring topics because every question follows the same 3 steps: grams to moles, apply the mole ratio, moles back to grams. Once you practise 8 to 10 problems it becomes almost automatic. NEET asks 1 to 2 questions from this area most years.
The mole ratio comes from the coefficients in the balanced equation and relates particles. The mass ratio relates the actual weights and is found only after multiplying moles by molar masses. You must use the mole ratio for the calculation; the mass ratio is just the final result.
Yes. Without a balanced equation the coefficients are wrong, so the mole ratio is wrong. Always balance first, then read the ratio. This follows the Law of Conservation of Mass.
In a simple stoichiometry problem one substance is given and everything else follows from it. In a limiting reagent problem TWO amounts are given, and you must first find which one runs out; only that one controls the product. Simple stoichiometry is the base skill you use inside limiting reagent problems.
