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Describe how the knowledge of the percent composition by mass of an unknown compound can help us identify the compound.

Percent composition leads directly to the empirical formula

Chemistry 101

Chapter 3

Mass Relationships in Chemical Reactions

Chemical reactions and Stoichiometry

Drexel University

University of Maryland - University College

University of Kentucky

Brown University

Lectures

04:02

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity. Chemical reactions are usually characterized by a chemical change, and they yield one or more products after the reaction is complete. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions. Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms.

08:02

In chemistry, a combination reaction is a chemical reaction in which two or more reactants combine to form more than one product. In a decomposition reaction, one reactant splits into two or more products.

02:07

Explain how the determinat…

07:14

How can percentage composi…

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What information does perc…

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Explain how percent compos…

02:51

Describe how to determine …

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Explain what is involved …

06:48

Explain how the informatio…

07:02

Explain how you can use th…

02:36

00:35

What is mass percent compo…

so knowledge of our percent composition can be very valuable to help us identifying unknown compound. So first, let's just walk through how you get the percent composition by mass. So if we're starting out with H 20 we know that the molar masses 18 grams from all and that the molar mass of the individual H. Adams are one gram per mole in the individual oxygen atom is 16 grams from all. Now, all you do to find the percent composition is take the ratio of the molar mass of the individual Adam. So for hydrogen, there's two of them. So we have two grams from all over the total over 18 grams from all, and we get 11% now, the same thing for oxygen. One oxygen atom is 16 years from all divided by the total 18 grams from all is 88%. So now we can use thes percents to help us find an empirical formula. And how you do this is we can just assume that I just say we assume we have 100 grams of something of this compound and we know that our percent compositions, So if we multiply our 100 grams times 11% or the desperate, which is 110.111 We get about 11 11 grams of hydrogen. Okay, so now let's convert that into moles. So we know that hydrogen has ah molar mass of one gram per mole. So we know that in our unknown compound there is 11 moles, so that's great. Now let's do the other one. So we still 100 grams so 100 times point a equals 88 and then eat eight grams and we're going convert that into I'm all. So now for oxygen, we know that there's 16 grams per mole. So when we calculate this out, we get 5.5 moles. So no, the empirical formula empirical formula is the smallest hold on the whole number ratio of the atoms in the molecule. So the smallest ratio, um, between these two, so you can divide both by 5.5. That's one equal one that's unequal to Now we know that there are two ages and there's 10 you put that the other we get h 20 So that is how we can move from our percent compositions all the way to our empirical formula down here.

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