How does dalton theory explain law of constant proportionals.

Question

Law of chemical combination."atoms and molecules"

Answer 1

Hi Aditya!

Great question! Dalton's atomic theory helps explain the Law of Constant Proportions, also called the Law of Definite Proportions.

First, what does the law mean?

The Law of Constant Proportions states that a pure chemical compound always contains the same elements combined in the same proportion by mass, regardless of where the compound comes from or how it is prepared.

How does Dalton's atomic theory explain this?

According to Dalton's atomic theory:

1. All matter is made up of tiny particles called atoms.
2. Atoms of a given element have a characteristic mass.
3. Atoms of different elements combine in fixed, simple whole-number ratios to form compounds.

Because the atoms combine in fixed ratios and have characteristic masses, the mass proportions of the elements in a particular compound remain constant.

Let's look at an example: Water (H₂O).

Every water molecule contains:

1. 2 hydrogen atoms
2. 1 oxygen atom

The approximate atomic masses are:

Hydrogen = 1 atomic mass unit (amu)

Oxygen = 16 atomic mass units (amu)

Therefore, the mass ratio of hydrogen to oxygen in water is:

Hydrogen : Oxygen = (2 × 1) : (1 × 16)

= 2 : 16

= 1 : 8

This means that pure water always contains hydrogen and oxygen in a mass ratio of 1:8, regardless of whether the water comes from a river, rainfall, or a laboratory.

For example, 9 grams of pure water contains approximately 1 gram of hydrogen and 8 grams of oxygen.

Final Answer:

Dalton's atomic theory explains the Law of Constant Proportions because atoms of different elements combine in fixed whole-number ratios to form a particular compound. Since atoms have characteristic masses, the elements in that compound are always present in a definite proportion by mass.

Easy way to remember:

Fixed number of atoms + Fixed atomic masses = Fixed mass proportions!

Hope this helps!

Michael