Question

Identify the mass law that each of the following observations demonstrates, and explain your reasoning: (a) A sample of potassium chloride from Chile contains the same percent by mass of potassium as one from Poland. (b) A flashbulb contains magnesium and oxygen before use and magnesium oxide afterward, but its mass does not change. (c) Arsenic and oxygen form one compound that is 65.2 mass \(\%\) arsenic and another that is 75.8 mass \(\%\) arsenic.

Short answer

Observation (a) demonstrates the Law of Definite Proportions, (b) demonstrates the Law of Conservation of Mass, and (c) demonstrates the Law of Multiple Proportions.

Step-by-step solution

Observations

Carefully read through each observation and identify key details that suggest certain mass laws.

Observation (a)

A sample of potassium chloride from two different locations contains the same percent by mass of potassium. This suggests that the composition of potassium chloride is constant everywhere, demonstrating the Law of Definite Proportions or Law of Constant Composition.

Observation (b)

A flashbulb's contents include magnesium and oxygen before use and magnesium oxide afterward, but the total mass remains unchanged. This demonstrates the Law of Conservation of Mass which states that mass is conserved in a closed system during any chemical reaction.

Observation (c)

Two compounds consisting of arsenic and oxygen form with different mass percentages of arsenic. This indicates that arsenic and oxygen can combine in different ratios to form different compounds, demonstrating the Law of Multiple Proportions.

Key concepts

Law of Definite Proportions

The Law of Definite Proportions, also known as the Law of Constant Composition, states that a chemical compound will always contain its component elements in a fixed ratio by mass. This means that no matter where a sample of a substance is found, its chemical composition remains the same.

For instance, as illustrated in observation (a), potassium chloride from Chile has the same percent mass of potassium as potassium chloride from Poland.

• This tells us that potassium chloride always contains potassium and chlorine in the same proportion.
• This consistency in composition is the essence of the Law of Definite Proportions.

Understanding this law is crucial because it allows chemists to predict the mass ratios of elements in compounds.

Law of Conservation of Mass

The Law of Conservation of Mass states that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations. The mass of the products in a chemical reaction must equal the mass of the reactants.

In observation (b), a flashbulb before and after use exemplifies this law.

• Before use, the flashbulb contains magnesium and oxygen.
• After the flash, it contains magnesium oxide, but the overall mass remains the same.

Because the total mass does not change, we see the Law of Conservation of Mass in action. This principle ensures that in any closed system, mass is conserved and helps balance chemical equations accurately.

Law of Multiple Proportions

The Law of Multiple Proportions states that when two elements form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first element are simple whole numbers.

An example is seen in observation (c), where arsenic and oxygen form compounds with different mass percentages of arsenic: 65.2% and 75.8%.

• This indicates that arsenic and oxygen can create multiple compounds.
• The varying ratios of arsenic in these compounds adhere to the principle of simple whole numbers.

This law is significant because it reveals that elements can combine in different ways to form distinct compounds with consistent ratio patterns.

Chemical Reactions

Chemical reactions involve the rearrangement of atoms to transform reactants into products. During a chemical reaction, bonds between atoms are broken and new bonds form, resulting in different substances.

For example, in the flashbulb scenario from observation (b), magnesium reacts with oxygen to form magnesium oxide.

• Before the reaction, we have separate magnesium and oxygen gases.
• After the reaction, magnesium oxide is produced.
• Despite this transformation, the total mass remains constant due to the Law of Conservation of Mass.

Understanding chemical reactions and the laws governing them is foundational to studying chemistry, as these reactions are the means by which new substances are created.

Mass Composition

Mass composition refers to the percentage by mass of each element in a compound. Knowing the mass composition is essential for understanding a compound's chemical makeup.

For example, in both potassium chloride samples from Chile and Poland (observation (a)), the mass composition of potassium is the same.

• This consistency is a direct result of the Law of Definite Proportions.
• In contrast, different mass compositions of arsenic in arsenic and oxygen compounds (observation (c)) show the application of the Law of Multiple Proportions.

By analyzing mass composition, chemists can determine the formulas of unknown compounds and gain deeper insights into chemical structures.