Law of definite proportions | Atoms and Molecules | Chemistry | Khan Academy

Summary

The law of definite proportions, also known as the law of constant proportions, dictates that elements in a chemical substance are always present in a fixed ratio by mass. This principle is exemplified by water (H2O), where the ratio of hydrogen to oxygen atoms is consistently 2:1, regardless of the water source. The mass ratio of oxygen to hydrogen in water is 16:2, simplifying to 8:1. This law applies universally to all chemical substances, ensuring a consistent composition regardless of sample size or origin.

Takeaways

• 📚 The law of definite proportions states that in any chemical substance, the elements are always present in definite proportions by mass.
• 🔍 This law can also be referred to as the law of constant proportions, emphasizing the constancy of the ratio of elements in a compound.
• 💧 Using water (H2O) as an example, the law indicates that the ratio of hydrogen to oxygen atoms is always 2:1, regardless of the water source.
• 🌐 This principle applies universally, meaning the elemental ratio in water is consistent whether it's from a tap, ocean, or lake.
• 🔬 The law extends to the mass of elements, with each atom having a specific atomic mass, such as hydrogen at 1 AMU and oxygen at 16 AMU.
• ⚖️ In water, the mass ratio of oxygen to hydrogen is 16:2, which simplifies to 8:1, indicating the mass of oxygen is eight times that of hydrogen in any water sample.
• 🔑 This law is not limited to water; it applies to all chemical substances, including carbon dioxide, which also exhibits a constant mass ratio of elements.
• 🌿 The law is exemplified in the formation of carbon dioxide during combustion or exhalation, where the mass ratio of oxygen to carbon remains constant.
• 🧪 Understanding the law of definite proportions is fundamental to the study of chemistry, as it helps in determining the composition of compounds.
• 📈 This principle is also crucial for chemical analysis and the formulation of the law of conservation of mass in chemical reactions.

Q & A

What is the law of definite proportions?

The law of definite proportions states that in any given chemical compound, the elements are always present in a fixed ratio by mass.

How can the law of definite proportions be simplified for understanding?

The law can be simplified by referring to the elements' presence in a definite or constant ratio by mass, rather than using the term 'proportions'.

What is an example given in the script to illustrate the law of definite proportions?

Water (H2O) is used as an example, where the ratio of hydrogen to oxygen atoms is always two to one, regardless of the water sample's source.

How does the law of definite proportions apply to the mass of elements in a compound?

The law extends to mass by stating that the mass ratio of elements in a compound is constant. For example, in water, the mass ratio of oxygen to hydrogen is 16:2 or simplified to 8:1.

What does it mean for the mass ratio of oxygen to hydrogen in water to be 8:1?

It means that for every 8 units of mass of oxygen in a water sample, there is 1 unit of mass of hydrogen.

How does the atomic mass unit (AMU) relate to the law of definite proportions?

The atomic mass unit (AMU) is used to express the mass of atoms, and the law of definite proportions uses these values to establish the constant mass ratios in compounds.

Is the law of definite proportions applicable to other chemical substances besides water?

Yes, the law applies to all chemical substances, including carbon dioxide, where the mass ratio of oxygen to carbon is always constant.

What is the significance of the law of definite proportions in chemistry?

It is significant because it provides a fundamental understanding of the composition of chemical compounds and the consistent mass relationships between elements within them.

Can the law of definite proportions be used to determine the purity of a substance?

Yes, by analyzing the mass ratios of elements in a substance, one can determine if it matches the expected ratios, indicating purity or the presence of impurities.

How does the law of definite proportions differ from the law of constant proportions?

The terms 'definite proportions' and 'constant proportions' are often used interchangeably to describe the same law, which states that the mass ratios of elements in a compound are fixed.

What is the role of the atomic mass of elements in the law of definite proportions?

The atomic mass of elements is crucial as it determines the mass ratios in compounds, which are the basis for the law of definite proportions.