A sample of \[{\text{CaC}}{{\text{O}}_{{\text{3}}}}{\;}\] has \[{\text{Ca: }}{\mathbf{40}}\% \], \[{\text{C}} = {\mathbf{12}}\% \] and \[{\text{O}} = {\mathbf{48}}\% \]. If the law of constant proportions is true, then the mass of \[{\text{Ca}}\]in 5 g of \[{\mathbf{CaC}}{{\mathbf{O}}_{{\mathbf{3}}}}\;\] from another source will be:
A.$2.0{\text{ g}}$
B.$0.2{\text{ g}}$
C.$0.02{\text{ g}}$
D.$20.0{\text{ g}}$

Hint: To answer this question, you should recall the concept of law of constant proportion. The law of constant proportions states that chemical compounds are made from elements that are present during a fixed ratio by mass. This implies that any pure sample of a compound, regardless of the source, will always contain an equivalent element that is present within the same ratio by mass.

Complete Step by step solution:
Since, mass percentage of Ca in \[{\mathbf{CaC}}{{\mathbf{O}}_{{\mathbf{3}}}}\; = 40\% \]​. Using, law of constant proportion is true, sometimes called Proust's law, or law of constant composition states that a given compound always contains its component elements in fixed ratio and does not depend upon its source and method of preparation.
So, the mass percentage of \[{\text{Ca}}\] in 5gm \[{\text{CaC}}{{\text{O}}_{{\text{3}}}}{\;}\] will also be \[{\mathbf{40}}\% \].
Hence, \[{\mathbf{40}}\% \]of 5gm =$5 \times \dfrac{{40}}{{100}}$=${\text{2}}{\text{.0 g}}$
Therefore, we will conclude that the right answer to the present question is option A.

Note: You ought to realize different laws governing chemistry. The Law of Conservation of Mass: The mass of a closed system will stay constant over time, no matter the processes occurring inside the system. Mass can't be made or destroyed but, it is often rearranged and altered into differing types of particles. For example, when water is made from hydrogen and oxygen, atoms of every element aren't lost or created; just rearranged. The Law of Constant Composition: A compound always has an equivalent proportion of elements by mass. The Law of Multiple Proportions: If two elements combine to make quite one compound, then the masses of the weather that combine are given during a small whole number ratio.