Question

A chemist decomposes several samples of carbon monoxide into carbon and oxygen and weighs the resultant elements. The results are shown in the table. MISSED THIS? Read Section 1.2 $$\begin{array}{ccc}\text { Sample } & \text { Mass of Carbon (g) } & \text { Mass of Oxygen (g) } \\\\\hline 1 & 6 & 8 \\ \hline 2 & 12 & 16 \\\\\hline 3 & 18 & 24 \\\\\hline\end{array}$$ a. Do you notice a pattern in these results? Next, the chemist decomposes several samples of hydrogen peroxide into hydrogen and oxygen. The results are shown in the table. $$\begin{array}{ccc}\text { Sample } & \text { Mass of Hydrogen (g) } & \text { Mass of Oxygen (g) } \\\\\hline 1 & 0.5 & 8 \\\\\hline 2 & 1 & 16 \\\\\hline 3 & 1.5 & 24 \\\\\hline\end{array}$$ b. Do you notice a similarity between these results and those for carbon monoxide in part a? c. Can you formulate a law from your observations in a and b? d. Can you formulate a hypothesis that might explain your law in c?

Short answer

The law formulated from the observations is the law of definite proportions, which states that a chemical compound always contains exactly the same proportion of elements by mass. A hypothesis explaining this could be that compounds are composed of molecules with fixed numbers and types of atoms that determine the mass ratios.

Step-by-step solution

Identify the Pattern in Carbon Monoxide Decomposition

Look at the mass of the elements in the carbon monoxide (CO) samples. For every 6 grams of carbon, there are 8 grams of oxygen. This ratio remains constant across samples: Sample 2 has twice the mass of carbon and oxygen as Sample 1, and Sample 3 has three times the mass. The constant ratio is 6g of carbon to 8g of oxygen, or simplified, 3g of carbon to 4g of oxygen.

Identify the Pattern in Hydrogen Peroxide Decomposition

Examine the table for the hydrogen peroxide (H2O2) samples. The mass of oxygen is 16 times the mass of hydrogen for all samples. This ratio is constant, meaning for every 0.5g of hydrogen, there is 8g of oxygen, which simplifies to a 1g hydrogen to 16g oxygen ratio.

Compare the Patterns

Compare the results of the carbon monoxide decomposition with those of the hydrogen peroxide. While the ratios of the elements are different between the two compounds, the constant ratio of elements within each compound indicates the law of constant composition, also known as the law of definite proportions—stating that a chemical compound always contains its component elements in fixed ratio.

Formulate a Law from Observations

Based on Steps 1-3, we can state the law of definite proportions: a chemical compound is always made up of the exact same proportion of elements by mass, regardless of the size or source of the sample.

Formulate a Hypothesis to Explain the Law

A potential hypothesis explaining the law could be that compounds are composed of molecules that have the same number and type of atoms. Each molecule of carbon monoxide contains one carbon atom and one oxygen atom, and each molecule of hydrogen peroxide contains two hydrogen atoms and two oxygen atoms. This would account for the fixed ratio of masses, as the mass is determined by the number and types of atoms it consists of.

Key concepts

Chemical Compound Composition

Understanding the composition of chemical compounds is essential for grasping the basics of chemistry. A chemical compound is a substance composed of two or more different elements that are chemically bonded together. The properties of a compound are distinct from those of its individual elements. For instance, water (H₂O) has very different properties from its constituent elements, hydrogen, and oxygen.

In the context of our exercise, examining samples of carbon monoxide (CO) and hydrogen peroxide (H₂O₂) reveals that these compounds maintain specific proportions in terms of the mass of the elements that constitute them. This consistency supports the foundational principle known as the law of definite proportions, which confirms that a chemical compound always contains exactly the same proportion of elements by mass. This leads to the conclusion that the chemical compounds have a fixed composition, which characterizes their unique identities in the realm of chemistry.

Mass Ratio of Elements

Diving deeper into the mass ratio of elements within a compound can reveal a lot about its chemical nature. The mass ratio refers to the proportion of the total mass of each element compared with the mass of the compound. It is often expressed as a simple ratio or a percentage. For example, in the exercise, we found that carbon monoxide has a mass ratio of carbon to oxygen of 3:4.

These ratios are the fingerprints of chemical substances and are crucial for identifying compounds. They are consistent regardless of the sample's size, a fact used to distinguish one compound from another. Moreover, the consistent mass ratio serves as an empirical basis for the formulas that chemists use to denote chemical compounds, such as CO for carbon monoxide, reflecting a carbon-to-oxygen ratio of 1:1 by a number of atoms, which translates into the mass ratio discovered in the exercise.

Atomic Structure of Molecules

At the core of the law of definite proportions is the atomic structure of molecules. Molecules are groups of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction. The atoms within a molecule are arranged in a specific geometric structure, and it is this arrangement that determines the fixed mass ratio seen in the exercises.

In the case of carbon monoxide and hydrogen peroxide, their molecules consist of a fixed number of carbon, hydrogen, and oxygen atoms. The mass ratio of the elements in these molecules reflects the masses of the individual atoms and how many of each type are present. For instance, hydrogen peroxide's molecule, H₂O₂, has twice the number of hydrogen atoms as oxygen atoms. Since the mass of an oxygen atom is significantly larger than that of a hydrogen atom, this molecular structure results in a consistent 1g of hydrogen to 16g of oxygen mass ratio for any given sample of pure hydrogen peroxide. The precision of this atomic structure and its contribution to the compound's characteristics highlights the intricate and orderly nature of the chemical world.