Which of the following pairs of compounds is an example of the law of multiple proportions? (A) CO and CO2, (B) NaCl and NaNO3, (C) Ca(OH)2 and CaCl2, (D) K2S and KF, or (E) H2O and H2S.
The law of multiple proportions is one of several mass laws that was discovered in the 18th century before the first atomic theory of matter. These mass laws are laws that hold true for all forms of matter, regardless of what elements they’re composed of. Today, these mass laws seem obvious now that we have an atomic theory to base our understanding of matter around. But at the time they were being discovered, they were quite mysterious.
The law of multiple proportions states that if we have two different elements, A and B, that can react to form different compounds, different masses of B that react with a fixed mass of A can be expressed in terms of simple whole number ratios. So say that we have a fixed mass of A, for example, the mass of one atom of A. It can combine with B in different ways to form different compounds, for example, AB and AB2.
So as we can see, the ratio of B in AB to B in AB2 is one to two, which is an example of the law of multiple proportions. As we can see from this example, this law is caused by the fact that atoms combine in different numbers to form different compounds. So in this question, we’re given different pairs of compounds and we’re tasked with identifying which one is an example of the law of multiple proportions.
So as we’ve just learned, an example of the law of multiple proportions will have different combinations of the same elements. If we look through all of our answer choices, almost all of them have different elements between the pair of compounds. Answer choice (A), CO and CO2, is the only one where we just have different combinations of the same two elements. In fact, it looks quite similar to our example if A was C and B was O. So CO and CO2 are an example of the law of multiple proportions.