Chemistry · Some Basic Concepts Of Chemistry · NEET
First find the molar mass of each compound. Molar mass = grams ÷ moles. Example: 0.1 mole of XY2 weighs 10 g, so its molar mass = 10 ÷ 0.1 = 100 g/mol. Do the same for the second compound. Now you have the total mass of each formula unit, which you will break into atomic weights.
Read the formula. XY2 means 1 atom of X and 2 atoms of Y. So molar mass of XY2 = X + 2Y. If that molar mass is 100, your equation is X + 2Y = 100. For X3Y2, molar mass = 3X + 2Y. This is why you need two compounds: each gives one equation, and two equations let you solve for two unknowns X and Y.
Use elimination. Take X + 2Y = 100 and 3X + 2Y = 180. Both have 2Y, so subtract the first from the second: (3X + 2Y) - (X + 2Y) = 180 - 100, giving 2X = 80, so X = 40. Put X = 40 back into X + 2Y = 100: 40 + 2Y = 100, so 2Y = 60 and Y = 30. Answer: X = 40, Y = 30.
For a compound, the molar mass (in g/mol) is just the sum of the atomic weights of all atoms in one formula unit. So for X3Y2 you add three X atomic weights and two Y atomic weights. There is no extra rule to memorise; you are only adding the masses of the atoms present.
Yes. This method works only when both compounds are made of the exact same two elements X and Y. That is what keeps X and Y as the same two unknowns in both equations. If the elements were different, you could not link the two equations together.
Line up the equations so one variable cancels. If the Y terms are not equal, multiply one full equation by a number to make them equal, then subtract. In NEET questions the numbers are usually chosen so a clean subtraction works, like both having 2Y.
Suppose the elements X and Y combine to form two compounds XY2 and X3Y2. When 0.1 mole of XY2 weighs 10 g and 0.05 mole of X3Y2 weighs 9 g, the atomic weights of X and Y respectively are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Molar mass = mass in grams ÷ number of moles. Do this for both compounds, write each molar mass as a sum of atomic weights (like X + 2Y), then solve the two equations together for the two atomic weights.
You have two unknown atomic weights (X and Y). To solve for two unknowns you need two independent equations, and each compound gives you one equation. One compound alone is not enough.
Atomic weight is for one element; molar mass is for the whole compound. In this method you first get the compound's molar mass from moles and grams, then break it into the elements' atomic weights.
NEET often gives 'moles + grams' data for two compounds of the same two elements and asks for atomic weights. It tests moles, molar mass, and simple two-equation solving together, so it is a fast scoring question if you set it up correctly.
