When $35$ g of wood was burnt in the air, it combined with $32$ g of oxygen to form $2$ g of ash and gas of carbon dioxide and water vapour. When the gas mixture was cooled, $18$ g of water was condensed. How much carbon dioxide was formed?

The amount of carbon dioxide formed was 65g.

The Law of conservation of mass helps us to know the unknown mass of the reactant or product by balancing the masses of reactants and products. Antoine Lavoisier explained the law of conservation of mass that it remains constant for the universe (system + surrounding).
Let the mass of carbon dioxide gas produced = x

As we know,

Mass of reactants = Mass of products
So, ⇒$(35 + 32)\mathit{g }= (2 +\mathit{ x})g$
⇒$\mathit{x }= 67\mathit{g }- 2\mathit{g }$
⇒$\mathit{x }= 65g$
So, 65g of carbon dioxide was formed.