Q.40 ml of methane is completely burnt using 80 ml of oxygen at room temperature. The volume of gas left after cooling to room temperature is

Stoichiometry (Greek: stoicheion = element, metron = measure) is the quantitative mole relationship between reactants and products fixed by a balanced chemical equation. The relationship is written with the symbol '≡' ("stoichiometrically equal to"): for C(s) + O₂(g) → CO₂(g), 1 mole of C ≡ 1 mole of O₂ ≡ 1 mole of CO₂.

The real power of stoichiometry is that moles, mass and gas volume are all freely inter-convertible:

• moles → mass: multiply by molar mass; mass → moles: divide by molar mass
• moles → gas volume: multiply by 22.4 L (at 273 K, 1 atm); volume → moles: divide by 22.4 L

General method for any stoichiometry problem:

1. Write and balance the chemical equation.
2. Convert whatever quantity you were given (mass or volume) into moles.
3. Use the balanced equation's mole ratio to find the moles of the substance you need.
4. Convert that mole answer back into whatever unit the question asks for (mass, volume, or leave as moles).

This is exactly the pattern used whether the question asks "how many moles of X are needed to produce Y moles of Z," "how much mass of product forms from a given mass of reactant," or "what volume of gas is released" -- read the mole ratio off the equation first, then convert units on either side of that ratio as needed.