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ISC Chemistry: Study Guide & Syllabus34 chapters | 248 lessons

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Jeremias Richter discovered the Law of Reciprocal Proportion in the late 18th century. In this lesson we will learn what this law is and how it is used by looking at some examples.

Do you like chocolate? Do you also like hot drinks? If you said yes to both questions, then you will probably like hot chocolate. We make comparisons such as this all the time throughout our lives. In the late 18th century **Jeremias Richter**, a German chemist, came up with an easy way to compare compounds and to see how two elements will combine to form another compound.

The **law of reciprocal proportions** says that if we know the proportion of elements in compound AB and the proportion of elements in compounds BC, we can determine the proportion of elements in compound AC. This law helped us to understand stoichiometry, or how we figure out the quantities of reactants and products in relation to their reactions.

Let's look at a few examples to make this law easier to understand.

Okay, so let's take a look at methane, CH4. Let's figure out the proportion of elements. The molecule weight of carbon is 12 g/mol, and the molecule weight of hydrogen is 1 g/mol. Since we have 4 atoms of hydrogen for every atom of carbon, the proportion is 12:4, which can be simplified down to 3:1.

Now, let's look at water, H2 O. The proportion of elements is 16:2, or 8:1 (oxygen has a molecular weight of 16).

So, methane and water both contain a hydrogen and one other element. According to this law if we combine carbon and oxygen (the other element in both compounds) it should be in a ratio of 3:8, or a simple multiple of that ratio.

We get 3:8 because, in methane, carbon is a ratio of 3, and in water, oxygen is a ratio of 8. Let's see if this is true, when carbon and oxygen combine they form carbon dioxide, CO2, which has a proportion of 12:32. This is equal to 3:8, exactly as we predicted!

Let's take a look at another example, starting with sodium chloride, NaCl. Sodium has a molecular weight of 23 g/mol, and chloride has a molecular weight of 35 g/mol. So the ratio is 23:35.

Now let's look at hydrochloric acid, HCl, where the ratio is 35:1. If we combine sodium and hydrogen, we would expect to see a ratio of 23:1. Yep - when these combine we form sodium hydride, NaH, with the ratio of 23:1.

Now, let's look at an example where the 3 compounds are a simple multiple of the expected ratio. Copper oxide, CuO, has a ratio of 63.5:16. Copper sulfide, CuS, has a ratio of 63.5:32. We would expect sulfur and oxygen to combine to form a ratio of 32:16 or 2:1.

Sulfur and oxygen combine to form sulfur dioxide, SO2. So the ratio is 32:32, or 1:1. We simply need to multiply 1 by 2 in order to get the expected ratio, so it is a simply multiple of the expected ratio.

When we stop and think about it, this makes sense, because each element has a set molecular weight and will always combine with whole numbers. But, at the time this was a big step in helping scientists understand how compounds combine with each other.

In the 18th century, **Jeremias Richter** realized something interesting about how compounds combine and originated the **law of reciprocal proportions**. This states that when we have two different compounds that share an element, they will combine in the same ratio (or a simple multiple of that ratio) to each other when the two different elements are combined.

This law helped us to understand stoichiometry, or how we figure out the quantities of reactants and products in relation to their reactions. Today we understand this law makes sense because each element has a set molecular weight and each element combines in whole numbers to make a compound.

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ISC Chemistry: Study Guide & Syllabus34 chapters | 248 lessons

- Accuracy vs. Precision in Chemistry: Definitions & Comparisons 3:41
- What Is Dimensional Analysis in Chemistry? - Definition, Method & Practice Problems 9:36
- Early Atomic Theory: Dalton, Thomson, Rutherford and Millikan 6:35
- The Law of Conservation of Mass: Definition, Equation & Examples 5:24
- The Law of Definite Proportions: Definition & Examples 5:36
- Law of Multiple Proportions: Definition & Examples 6:06
- Law of Reciprocal Proportion: Definition & Examples
- Go to Basic Concepts of Chemistry

- Go to Isomerism

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