The wood in this pile will be reduced to this much ash when it is burned. What happens to the rest of the mass from the wood?
Wood is mostly made up of cellulose, a very long molecule that is really a chain of sugar molecules. The plant makes that sugar from carbon dioxide and water, with energy from sunlight in the process of photosynthesis. When you burn the wood, you get back the energy, mostly in the form of light and heat. That energy was stored in the chemical bonds, and is released when those bonds are broken. You also get back carbon dioxide and water vapor, both of which are gases. Those gases go up the chimney. The gray ash that is left behind is mostly the material that the tree took out of the soil. If you combined the mass of the ash with the mass of all the carbon dioxide and water vapor, it would equal the mass of the original pile of wood. None of the mass is destroyed or converted into energy.