[reyaz01]

3 mols of sulphur dioxide react with 2 mols of O₂ to guve SO₃..(1) which is d limiting reagent..(2) what is the max amt of SO₃ that can be formed...(3) how much of 1 of the reactants remains unreacted...

OK, I attempted the question and wrote the balanced equation:
2SO₂ + O₂  :rarrow:2SO₃

So I identified O₂ as the limiting reagent, and used that for further calculation
the ratio of O₂:SO₃
from the balanced equation
is 1:2
therefore

if 2 moles of Oxygen was used
the new ratio will be
2:4

so 4 moles of SO₃ can be formed?

But then how much moles of the other reactant will remain excess? 1 mole?

[Schrödinger]

So I identified O2 as the limiting reagent

Are you sure? The ratio of SO2 to O2 for complete reaction is 4:2, but we only have only 3:2...

[reyaz01]

OK im not sure then, can you help me out?

[stewie griffin]

Using dimensional analysis and the stoichiometry of the balanced chemical equation, calculate how many moles of product each amount of  reactant could give. Which ever reactant would provide you with the least amount of product is your limiting reagent then.
FYI you can't just look at the chemical equation and know what is the limiting reagent. I have college sophomores in organic lab that still try to find limiting reagent by just looking at the chemical equation and guessing. Just do as I said above and you will always get the correct answer.

[reyaz01]

ok now im more confused than ever, i thought the limiting reagent is the one present in the least and therefore will directly affect the amount of product obtained.

[Borek]

ok now im more confused than ever, i thought the limiting reagent is the one present in the least and therefore will directly affect the amount of product obtained.

Not exactly, you should look at the stoichiometry of the reaction.

2H₂ + O₂ -> 2H₂O

When you have 1 mole of both oxygen and hydrogen hydrogen is a limiting reagent. Why? Because when you look at the stoichiometry 1 mole of hydrogen reacts with half mole of oxygen (excess oxygen present), but one mole of oxygen reacts with two moles of hydrogen (and we don't have that much).

Now imagine you have 0.75 mole of oxygen and 1 mole of hydrogen. Not much have changed - still hydrogen is a limiting reagent, even if it is oxygen that is in a lesser amount.