Question

State the main idea of the law of constant composition in your own words, using an example to illustrate your understanding of this important idea.

Short answer

The law of constant composition states that the mass ratio of elements in a compound is always constant, regardless of the compound's source or amount. In our example of water (H₂O), we calculated the mass of hydrogen and oxygen in one water molecule and found the mass ratio of hydrogen to oxygen to be 1:8. This ratio stays constant in any water sample, illustrating the importance of the law of constant composition in understanding chemical reactions and predicting product amounts.

Step-by-step solution

Understand the Law of Constant Composition

The law of constant composition states that the mass ratio of elements in a compound will always be constant, regardless of the source or amount of the compound. This means that in a specific compound, the elements will always combine in the same proportions. This is important because it helps us understand how chemical reactions occur and how to predict the amounts of products created in a chemical reaction.

Choose an Example

Let's use the example of water, H₂O, to demonstrate the law of constant composition. Water is a simple compound composed of two hydrogen atoms and one oxygen atom.

Calculate the Mass Ratio of Elements in Water

To show that the mass ratio of hydrogen to oxygen in water is constant, we need to determine the mass of each element in one molecule of water. To do this, we will use the atomic masses of hydrogen and oxygen. The atomic mass of hydrogen is approximately 1 atomic mass unit (amu), and the atomic mass of oxygen is approximately 16 amu. In a water molecule, there are two hydrogen atoms and one oxygen atom: Mass of hydrogen in H₂O = 2 × 1 amu = 2 amu Mass of oxygen in H₂O = 1 × 16 amu = 16 amu

Determine the Mass Ratio of Hydrogen to Oxygen

Now that we have the mass of hydrogen and oxygen in one water molecule, we can determine the mass ratio of hydrogen to oxygen: Mass ratio of hydrogen to oxygen = 2 amu (hydrogen) / 16 amu (oxygen) Simplify the ratio: Mass ratio of hydrogen to oxygen = 1 / 8

Verify that the Mass Ratio is Constant

The mass ratio of hydrogen to oxygen in water is 1:8. This will always be the case, regardless of the amount or source of water. If you take a water sample from a river, a lake, or the ocean, or even if you create water through a chemical reaction, you will find that the mass ratio of hydrogen to oxygen is still 1:8. This constant mass ratio demonstrates the law of constant composition. In summary, the law of constant composition states that the mass ratio of elements in a compound will always be constant. Using water as an example, we found that the mass ratio of hydrogen to oxygen is 1:8, which is consistent with the law of constant composition.

Key concepts

Chemical Composition

At the heart of chemistry lies the exploration of substances and how they are made up, which is where the term 'chemical composition' comes into play. Chemical composition refers to the identity and proportion of the elements that make up any particular compound.

For instance, water's chemical composition is universally known as H₂O, signifying that each molecule consists of two hydrogen atoms bonded to a single oxygen atom. The significance is that no matter where you encounter water, on Earth or beyond, its chemical composition remains unchanged: always two hydrogens for every oxygen.

Understanding chemical composition is crucial for studying and predicting the behavior of substances in various chemical reactions. It also becomes a foundation for more complex concepts in chemistry, such as the stoichiometry of reactions, where the precise amounts of reactants determine the possible yield of products.

Mass Ratio in Compounds

Diving deeper into chemical compositions, the 'mass ratio in compounds' is an essential concept that reveals the quantitative aspect of composition. It represents the proportion, by mass, of each element within a compound relative to the others.

The law of constant composition is beautifully exemplified here. Irrespective of the sample size, the mass ratio remains constant. Consider sodium chloride (table salt); its composition of sodium to chlorine is always 1:1 by atoms, but due to the differing atomic masses, the mass ratio is not 1:1. Here's where the concept of the atomic mass unit comes into play to provide a common ground for such comparisons.

Illustrating Mass Ratio with Common Salt

One atom of sodium (Na) has an atomic mass of approximately 23 amu, while one atom of chlorine (Cl) has an atomic mass of about 35.5 amu. Hence, the mass ratio of Na to Cl in sodium chloride is 23:35.5. This ratio holds true for any amount of sodium chloride, whether it's a pinch or a mountain.

Atomic Mass Unit

The 'atomic mass unit' (amu) is a standard unit of mass that quantifies mass on an atomic or molecular scale. It's defined as one twelfth of the mass of a carbon-12 atom, which is roughly equal to the mass of one proton or neutron.

The use of amu allows chemists to convey the mass of atoms and molecules in a more comprehendible way. For example, when we say the atomic mass of hydrogen is approximately 1 amu, we're essentially saying that a hydrogen atom is about one-twelfth the mass of a carbon-12 atom.

Thinking in terms of amu simplifies the understanding of chemical reactions on a molecular level, especially when comparing the masses of different atoms. In reactions, we rely on the atomic mass to help balance equations and to calculate the amounts of each element that would react or be produced, which in turn is intrinsically linked to the mass ratio concept explained earlier.