THE COMPONENTS OF MATTER
Refer to the definitions of an element and a compound.
Unlike compounds, elements cannot be broken down by chemical changes into simpler materials.
different types of atoms; there is only one type of atom in an element.
Some elements, such as the Noble gases (He, Ne, Ar, etc.) occur as individual atoms. Many other elements, such as most
other nonmetals (O
, etc.) occur as molecules.
Mixtures have variable composition; therefore, the amounts may vary. Compounds, as pure substances, have constant
composition, thus their composition cannot vary.
The tap water must be a mixture, since it consists of some unknown (and almost certainly variable) amount of
dissolved material in solution in water.
If you have no reliable way of determining the composition of a sample, you have no hope of knowing if its
composition is constant.
Generally no, the composition of a compound is determined by the elements used, not their amounts. If too much of one
element is used, the excess will remain as unreacted element when the reaction is over.
These two experiments demonstrate the Law of Definite Composition. In both cases, the ratio of
(g Cu reacted)/(g I reacted) is 0.50, so the composition is constant regardless of the method of preparation. They also
demonstrate the Law of Conservation of Mass, since in both cases the total mass before reaction equals the
total mass after reaction.
a) 2.34 g compound - 2.03 g lead = 0.31 g sulfur
b) Mass fraction Pb = 2.03/2.34 = 0.86752 =
Mass fraction S = 0.31/2.34 = 0.13248 =
c) Mass % Pb = 0.868 fraction x 100 = 86.752 =
Mass % S = 0.13 fraction x 100 = 13.248 =
Mass of compound = 63.94 g cesium + 61.06 g iodine = 125.00 g compound
Mass of cesium =
63.94 g cesium
125.00 g compound
= 19.83163 =
19.83 g cesium
Mass of iodine =
61.06 g iodine
125.00 g compound
= 18.9384 =
18.94 g iodine
77.6 mass % Xe
22.4 mass % F
= 3.4643 = 3.46
63.3 mass % Xe
36.7 mass % F
= 1.7248 = 1.72
= 2.0116 = 2.01 / 1.00
are in a 2:1 ratio, supporting the Law of Multiple Proportions.