Question

All samples of carbon dioxide contain carbon and oxygen in the mass ratio of 3:8. This is in agreement with the law of:
A) Conservation of mass
B) Constant proportion
C) Multiple proportions
D) Gaseous volumes

Answer 1

Hint: We know that any pure sample of a compound, no matter the source, always consists of the same elements that are present in the same ratio by mass.

Complete answer:
We know that carbon Dioxide always has a formula of \[C{O_2}\], independent of its source. So, we can easily calculate the ratio by mass of the two elements that constitute this compound, which are carbon and oxygen. This is in agreement with the Law of Constant proportion or Law of Definite Proportion which was proposed by Joseph Proust.
Since each molecule of \[C{O_2}\] has 1 Carbon atom and 2 Oxygen atoms, we can easily calculate the ratio by mass of these two elements if we know the individual masses of the atoms of these two elements.
Carbon atom has an atomic mass of \[12{\text{ }}u\] and an Oxygen atom has an atomic mass of \[16{\text{ }}u\]. So, the ratio can be calculated as:
\[\dfrac{{Mass{\text{ }} of {\text{ }}C{\text{ }}}}{{Mass{\text{ }}of{\text{ }}O}} = \dfrac{{12}}{{2 \times 16}} = \dfrac{{12}}{{32}} = \dfrac{3}{8}\]
So, we can see that every pure sample of carbon dioxide would always contain carbon and Oxygen in the mass ratio of 3:8.

So, the correct option would be (B).

Additional information: There is another law that is concerned with mass ratios that is the law of Multiple Proportions. According to this law, if two elements combine to form more than one compound, then, the ratios of the masses of the second element which combine with a fixed mass of the first element are always the ratios of small whole numbers.

Note: Students often mix-up the mass ratio of elements in a compound with the ratio of the number of atoms of different elements present in a molecule of the compound. Like, in \[C{O_2}\], the ratio of the number of atoms of C and O is 1:2 while the mass ratio of C and O is 3:8.