Question

Prelab for Experiment 4: The Atomic Theory of Matter 1. Demonstrate the Law of Multiple Proportions with the following data: a. Iron forms two compounds with chlorine, compound X and compound Y. When 3.00 g of compound X was analyzed, it was found that 1.68 g of CI combined with 1.32 g of Fe. When 5.00 g of compound Y was analyzed, it was found that 3.28 g of Cl combined with 1.72 g of Fe. To illustrate the Law, one needs to know how much of one element combines with the same mass as the other element in each compound, but the data above give different masses for both Fe and Cl. One way to standardize the data is to find out how much of one element, say Cl, combines with (if 1.75 g of Cl combines with 1.25 g of Fe, how many g of Cl would combine with 1.00 g of Fe...") Do this for both compounds, X and Y exactly 1 g of the other, say Fe. This can be done by a simple proportion method

Answer 1

The Law of Multiple Proportions states that if 2 elements form more than one compound between them, if the mass of one of the elements is fixed, the masses of the other element will be in a ratio of small whole numbers.

In this case, the two elements are Fe and Cl, and the 2 compounds are X and Y. Let us fix the mass of Fe in 1.00 g and analyze what happens with the masses of Cl.

For X:

$1.00gFe.\frac{1.68gCl}{1.32gFe} =1.27gCl$

For Y:

$1.00gFe.\frac{3.28gCl}{1.72gFe} =1.91gCl$

The ratio mCl in X/mCl in Y is 1.27g/1.91g = 0.664 ≅ 2/3. This is a ratio of small whole numbers, which proves the Law of Multiple Proportions.