When paper is burned, the mass of the remaining ash is less than the mass of the original paper because Some of the products of the reaction were given off as a gas.
The postulate of the law of conservation of mass is that mass can not be created or destroyed rather it can be transformed from one form to another.
This means that when ash is left behind after burning paper, the total mass should be the same as was obtained before combustion but it rather weighs less than paper itself. This should never be interpreted to mean that mass was "lost".
Here is what happens, paper is an organic material and burning it in air yields CO2 gas which escapes into the atmosphere. This explains why; when paper is burned, the mass of the remaining ash is less than the mass of the original paper.
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