Baggue weight 2.0g

Chemical A weight 0.6g

Chemical B weight 1.0g

Chemical C weight 0.5g

Total weight 4.1g

And at last, we got 4.1 g. I think that is because we close the baggie immediately once we pour the C into the baggie. When the chemical change happened, all the heated air was kept in the baggie, so we get the mass the same as we caculated.