Burning a wood is a chemical change which produces many products. According to the law of conservation of mass, matter is neither created nor destroyed in a chemical reaction.
This implies that the quantity of the product and reactants will remain the same before and after the reaction.
When we write a balanced reaction equation for this process, we find that the number of moles are conserved and the same.
So, this is replicated in the mass of the combining species and the product they form.
