Law of Definite Proportions - Video Tutorials & Practice Problems
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"Different samples of a pure chemical compound always contain the same proportions of elements by mass."
Proust's Law of Definite Proportions
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Law of Definite Proportions
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Let's examine the law of definite proportions. In 17/99, the French chemist Joseph L. Probst originated the law of definite proportions. Because of its immense contributions to it, it's 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 c02, And I take a sample of in another city, London, England, and I suspect that's c02. If we're following the law of different proportions, both samples should have the same mass ratio. Now for the mass ratio, we're gonna place the element with the larger mass on top. Now, going back to our whole idea of CO2 in 2 different cities. Remember, CO2 itself is composed of 1 carbon and 2 oxygens. And we know that if we look at the periodic table, the atomic mass of carbon is 12.01 grams per mole, and the mass of oxygen is around 16 grams 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 c02. So carbon itself contributes 12.01 grams total, and oxygen contributes 32 grams total. Now using the mass ratio, we place the larger mass on top, which is the 32 grams 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 2.6 6 oxygen for every one carbon. This would be our mass ratio. And if I were examining 2 samples of c02, 1 from New York City and 1 from London, if they both were indeed c02, they both should give me back the same exact mass ratio. That's what the law of definite proportion 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. Alright. 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.
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Law of Definite Proportions Example 1
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Here in this example, it says using the law of definite proportions, illustrate why no two and and O represent different compounds. All right. So they want us to use the law of definite proportions. So that basically means we need to determine their mass ratios. So if we look at no two, which is nitrogen dioxide, so no two here has one nitrogen and two oxygens. The atomic masses of these elements based on the periodic table is 14.01 g per mole for the nitrogen and 16 g per mole for the oxygen. So that's 14.01 g per mole for N and 32 g per mole. For, for o remember for the mass ratio, you put the larger mass above the smaller one. So its mass ratio would be 32 g per mole for O divided by 14.01 g per mole or nitrogen. That gives us 2.284 for its mass ratio. I know it is just one nitrogen, one oxygen. So that's one nitrogen, one oxygen. Again, they have the same exact atomic masses from the periodic table. So plug those in, that'd be 14.01 g per mole for nitrogen and 16.0 g per mole for oxygen place the larger mass above the smaller one. So 16.0 g per mole for O and 14.01 g per mole for N. So here when we do that, we're gonna get 1.142 the fact that they have different mass ratios is a way of proving through the law of de definite proportions that they represent different compounds, right? So that's what we'd say in terms of this particular example question.
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Law of Definite Proportions Example 2
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Now, in this video, we're gonna talk about proportions and how it relates to the law of definite proportions. Now, 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, right? So if we take a look at 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 g of silver and 2.80 g. 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 g of silver. All right. So to solve this, we're gonna say we have mass ratio one. And if we follow the law of ma of definite proportions and it's for the same substance, then the mass ratio too should be equal to each other, right? So mass ratio, one mass ratio two 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 g. Silver on our periodic table is A G divided by 2.80 g. Oxygen equals. Now we don't know the grams of oxygen for the mass ratio too. So we're gonna make that X but we do know the amount of silver which is 6.63 g, silver. We're gonna use proportions here basically to solve for our missing variable. So cross multiply 3.40 and X and then cross multiply 2.80 and 6.63. When we do that, we'd have 3.40 X equals now multiplying 2.80 times 6.63 would give us 18.564 divide both sides here by 3.40 X equals 5.46 grants. And this would be the mass of our oxygen. So this would be our final answer.
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Problem
Problem
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.