Question

A piece of chalk (CaCO3) has an initial mass of 43.5 grams. The massof the chalk decreased to 39.6 grams after it was used. Which of these is the closest value to the amount of chalk used?

Answer 1

3.9 grams due to law of conservation of mass
Total - left = used

Answer 2

The mass of chalk in this case is 3.90g.

According to the law of conservation of mass, mass can neither be created nor destroyed hence the mass before reaction is equal to the mass after reaction.

Any loss in mass will therefore reflect the mass of chalk used. Therefore, the mass of chalk used in this case is 3.90g.

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