How much mass does a slice of bread have?

This question is difficult because you don't have a succinct definition for "Bread". If you make dough, and use liquids (milk, water, the water in eggs, etc.), then when you bake the bread, some of the water will evaporate. This will reduce the mass of the loaf of bread, because the water has gone elsewhere, but the water itself will retain their own mass. So yes -- when you bake bread, evaporation reduces the mass of the dough you started with.