Law of Definite Proportions - Online Tutor, Practice Problems & Exam Prep

The Law of Definite Proportions, also known as Proust's Law or the Law of Constant Composition, states that a chemical compound always contains exactly the same proportion of elements by mass. This foundational chemical principle means that the mass ratios—fractions representing the masses of each element in a compound—remain constant regardless of the sample's source. For example, carbon dioxide (CO₂) from New York City will have the same mass ratio as CO₂ from London, reflecting the consistent mass contributions of carbon (12.1 g/mol) and oxygen (16 g/mol). To determine the mass ratio, the mass of the element with the higher contribution is placed on top. In the case of CO₂, the mass ratio is approximately 2.66, indicating 2.66 grams of oxygen for every gram of carbon. This ratio is crucial for comparing different samples and confirming their identity as the same chemical compound. Understanding and applying this law is essential for analyzing and verifying chemical compositions.

Law of Definite Proportions

Let's examine the Law of Definite Proportions. In 1799, the French chemist Joseph L. Proust originated the Law of Definite Proportions because of its immense contributions to it. It is sometimes referred to simply as Proust's Law, but it also goes by the law of constant or definite composition. The law itself uses what we call mass ratios. These mass ratios are fractions or proportions of elements by mass.

The whole concept behind the law of definite proportions is that different samples of a pure chemical compound always contain the same proportions of elements by mass. So let's say, for example, that I take a sample from New York City and I suspect that it's CO₂, and I take a sample in another city, London, England, and I suspect that CO₂. If we're following the law of definite proportions, both samples should have the same mass ratio. Now for the mass ratio, we're going to place the element with the larger mass on top.

Now going back to our whole idea of CO₂ in two different cities. Remember, CO₂ itself is composed of one carbon and two oxygen, and we know that if we look at the periodic table, the atomic mass of carbon is 12.01g per mole and the mass of oxygen is around 16g per mole. When you multiply the number of each element by its atomic mass, we'll see how much of it contributes to the overall mass of CO₂. So carbon itself contributes 12.01g total and oxygen contributes 32g total.

Now using the mass ratio, we place the larger mass on top, which is the 32g of oxygen, and the smaller mass on the bottom. When we divide those two numbers, it gives me approximately 2.66. What that 2.66 is telling me is that we have 2.66 oxygen for every one carbon. This would be our mass ratio. And if I were examining two samples of CO₂, one from New York City and one from London, if they both were indeed CO₂, they both should give me back the same exact mass ratio.

That's what the law of definite proportions hinges on. If we know the mass ratio of a given sample and we're examining it against an unknown sample, I use this law to determine if they're the same sample. All right, now we get the idea behind the law of definite proportions, so let's move on. Let's talk about calculations and different types of problems associated with the Law of Definite Proportions.

Law of Definite Proportions Example 1

Here with this example, it says, using the law of definite proportions, illustrate why NO₂ and NO represent different compounds. Right. So, they want us to use the law of definite proportion. That basically means we need to determine their mass ratios. If we look at NO₂, which is nitrogen dioxide, NO₂ has 1 nitrogen and 2 oxygens. The atomic masses of these elements based on the periodic table are 14.01 grams per mole for Nitrogen and 16 grams per mole for Oxygen. So, that's 14.01 grams per mole for N, and 32 grams per mole for O. Remember, for the mass ratio, you put the larger mass above the smaller one. So, its mass ratio would be 32 grams per mole for O divided by 14.01 grams per mole for Nitrogen. That gives us 2.284 for its mass ratio. NO is just 1 nitrogen, 1 oxygen. So, that's 1 nitrogen, 1 oxygen. Again, they have the same exact atomic masses from the periodic table, so plug those in. So that'd be 14.01 grams per mole for Nitrogen and 16.0 grams per mole for Oxygen. Place the larger mass above the smaller one. So 16.0 grams per mole for O and 14.01 grams per mole for N. So here, when we do that, we're going to get 1.142. The fact that they have different mass ratios is a way of proving through the law of definite proportions that they represent different compounds. Right? So, that's what we'd say in terms of this particular example question.

Law of Definite Proportions Example 2

Now in this video, we're going to talk about proportions and how it relates to the law of definite proportions. You can determine the unknown amount of an element if you know the mass ratio and the mass of the other element. This is accomplished through the use of a proportion. Alright. So, if we take a look here at this example question, it says, a substance is found to contain only silver and oxygen. An examination of this substance finds that it contains 3.40 grams of silver and 2.80 grams oxygen. Based on the law of definite proportions, how many grams of oxygen should another sample of this substance possess if it has 6.63 grams of silver? Alright. So to solve this, we're going to say we have mass ratio 1. And if we're following the law of definite proportions, and it's for the same substance, then the mass ratio 2 should be equal to each other. Alright. So mass ratio 1, mass ratio 2 should be equal to one another based on the law of definite proportions. Remember, we place the larger mass over the smaller one. So here, 3.40 is the larger number, 3.40 grams, Silver on our periodic table is Ag, divided by 2.80 grams oxygen equals now we don't know the grams of oxygen for mass ratio 2, so we're going to make that x. But we do know the amount of silver, which is 6.63 grams silver. We're going to use proportions here basically to solve for our missing variable. So, cross multiply 3.40 in x, and then cross multiply 2.80×6.63. When we do that, we'd have 3.40x equals. Now multiplying 2.80 times 6.63 would give us 18.564. x = 18.564 3.40 = 5.46 grams . And this would be the mass of our oxygen. So, this would be our final answer.

The formula for sodium hydroxide is NaOH. A sample of NaOH weighing 18.1124 is found to be comprised of 10.41 g Na and .456 g H. Determine the grams of oxygen most likely to be found in a sample that weighs 25.360 g.

• A. 10.1 g 
• B. 8.29 g
• C. 15.6 g
• D. 16.0 g

Here’s what students ask on this topic:

What is the law of definite proportions?

The law of definite proportions, also known as Proust's Law, is a fundamental principle in chemistry that states that a chemical compound always contains exactly the same proportion of elements by mass. This means that no matter the source of a compound or how it was prepared, the ratio of the masses of the elements in the compound will always be the same. For example, water ($H_{2}O$) is composed of hydrogen and oxygen, and it always contains hydrogen and oxygen in a mass ratio of roughly 1:8, respectively. This law is foundational for the understanding of chemical compounds and reactions, as it helps to predict the outcomes of chemical combinations and supports the concept that compounds are composed of fixed ratios of atoms.

Who discovered the law of definite proportions?

The Law of Definite Proportions, also known as Proust's Law, was discovered by the French chemist Joseph Louis Proust. He formulated this fundamental chemical principle between 1797 and 1804. The law states that a chemical compound always contains exactly the same proportion of elements by mass. This means that no matter the source of a chemical compound or how it was prepared, it will always be composed of the same elements in the same proportion by mass. Proust's work was initially controversial but was later confirmed by other scientists and became a cornerstone of modern chemistry, underpinning the stoichiometric principles of compounds.

How does the law of definite proportions apply to compounds?

The law of definite proportions, also known as Proust's Law, states that a chemical compound will always contain the exact same proportion of elements by mass, regardless of the size or source of the compound. For example, water (H2O) is composed of hydrogen and oxygen, and it always contains 2 grams of hydrogen for every 16 grams of oxygen, which gives a fixed mass ratio of 1:8. This means that no matter how much water you have or where you get it from, the proportion of hydrogen to oxygen by mass will remain constant.

In practice, this law means that when compounds are formed, the elements combine in certain fixed ratios and not in any arbitrary amounts. This is a fundamental principle in chemistry that helps chemists understand the composition of substances and predict how they will react with one another. It is also essential for the stoichiometric calculations that are crucial for preparing compounds and for reactions in the lab.