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Why Are Moles Used In Chemistry

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The Mole And Avogadro’s Constant

How to Use the MOLE in Chemistry
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  • The number of moles in a system can be determined using the atomic mass of an element, which can be found on the periodic table. This mass is usually an average of the abundant forms of that element found on earth. An element’s mass is listed as the average of all its isotopes on earth.

    Avogadro’s Constant

    One mole of oxygen atoms contains \ oxygen atoms. Also, one mole of nitrogen atoms contains \ nitrogen atoms. The number \ is called Avogadro’s number ) orAvogadro’s constant, after the 19th century scientist Amedeo Avogadro.

    Each carbon-12 atom weighs about \ therefore,

    Why Do Chemists Need The Mole Concept

    The Mole concept

    Chemists need the mole concept to bridge the gap between the microscopic world of atoms to the macroscopic world of humans. As you know, the molecular level consists of particles that are invisible to us. Because of this, chemists cant count or weigh these individual particles on a scale.

    Molecular Weight And Atomic Weight

    Atomic weight is useful for determining a substances molecular weightthe average combined weight of a molecules individual atoms. Like atomic weight, molecular weight is an averaged weight, based on the relative abundance of each atoms isotopes.

    Lets go back to carbon dioxide for an example of how this works. Each CO2molecule has one carbon atom and two oxygen atoms. Therefore, CO2s molecular weight can be calculated by adding the atomic weights of one carbon atom and two oxygen atoms:

    MW CO2 = + MW CO2 = + MW CO2 = 44.0098 amu

    Because molecular weight and molar mass are linked, weve also just determined the mass of one mole of CO2: one mole of CO2 has a molar mass of 44.0098 grams per mole.

    Comprehension Checkpoint

    To calculate molecular weight, it helps to know _____.

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    Key Concepts And Summary

    A convenient amount unit for expressing very large numbers of atoms or molecules is the mole. Experimental measurements have determined the number of entities composing 1 mole of substance to be 6.022 × 1023, a quantity called Avogadros number. The mass in grams of 1 mole of substance is its molar mass. Due to the use of the same reference substance in defining the atomic mass unit and the mole, the formula mass and molar mass for any substance are numerically equivalent .

    Why We Use Moles


    Why don’t we simply stick with units like grams ? The answer is that moles give us a consistent method to convert between atoms/molecules and grams. It’s simply a convenient unit to use when performing calculations. You may not find it too convenient when you are first learning how to use it, but once you become familiar with it, a mole will be as normal a unit as, say, a dozen or a byte.

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    Estimating The Numerical Value Of Avogadros Number

    The mole, the SI unit for substance, is a unit for counting, and its often said that we can think of the mole as like a dozen. A subtle difference, however is that a dozen is defined as a specific number of items, but a mole is defined as whatever number of H atoms youd have if you weighed out 1.00794 g of H, .

    This usually isnt a problem, as these terms are normally encountered. If we bought a dozen donuts, wed want to know how many we gotis there one for each person, etc.. But if we weigh out a mole of Na and a mole of Cl , we probably dont care how many atoms we havewere just happy to know we have the same number of atoms of each, in order to make NaCl without any atoms of Na or Cl leftover.

    However, in some relatively rare circumstances, we may need to know how many atoms or molecules do we have when we weigh out a certain amount. For this, wed actually like to know a numerical value for Avogadros number. To get this value, someone has to measure how many H atoms are there in 1.00794 g of H, . To do that, someone has to measure the mass, in grams, of a single atom.

    So, based on current best measurements, we can write:

    1 Mole of things = Avogadros number of them

    Avogadros number = 6.0221418 x 1023 = 6.022 x 1023 approximately

    Remember: this value, 6.022 x 1023, refers to entities/particles/atoms/ molecules/ions whatever you have a mole ofper mole, just like dozen can be for muffins, donuts, cookies, etc.

    Figure 1.Figure 2.Figure 3.

    The Mole Is Used Extensively In Chemistry Why Not Elsewhere

    In chemistry, the unit “mole” is used in many places, and is an important tool when working with quantities.

    I have not seen it used elsewhere–for example, we never count people or TVs with the unit.

    What makes the mole so useful in chemistry, and why isn’t it used elsewhere?

    Because the mole is not a convenient basis for people and TV sets times the population of earth, or put another way the earth contains a human population concentration of approximately 12 femtomoles/planet), and because there’s no connection back to atomic mass, which is what makes the mole useful in the first place.

    This was not part of your original question, but addresses Manishearth’s modifications:

    Because atoms and molecules react in discrete units , the number of particles is more fundamental than the masses of those particles. The mole is useful in chemistry because it is defined such that the mass of Avogadro’s number of particles of a substance is equal to the sum of the numerical values of the atomic masses of its constituent elements. The value of the mole is calibrated such that the molar mass of carbon-12 is exactly 12 grams per mole. On the periodic table you will note that the atomic masses given are non-integral . This is because the atomic masses represent a weighted average of isotopic masses, and because the proton and neutron masses aren’t quite equal.

    We don’t use atomic mass units outside chemistry, so we also have no need for the mole.

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    Definition Of The Mole And Avogadros Number

    The mole, or mol is a unit of measurement in chemistry, used to designate a very large number of molecules, atoms, or particles. This very large number is called Avogadros Number: 6.02214 x 1023, the number of units in a mole. These numbers are very important for telling us about the quantity of elements. A mole of a substance will end up being anywhere from a few grams of a substance, to hundreds of grams.

    What You Need To Know About The Mole An Important Chemistry Idea

    How To Use Moles – Part 1 | Chemical Calculations | Chemistry | FuseSchool
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    October 23rd marks Mole Day in the United States. Chemistry classes and chemistry enthusiasts celebrate this unofficial holiday with activities, cakes and jokes galore. Heres what you need to know.


    What is a mole?

    A mole is a unit of measurement in chemistry. Here is the official definition: One mole of something is equal to as many of that something as there are atoms in 12 grams of the isotope carbon-12.

    And how much is that exactly?

    Experiments have nailed down the number as 6.02 times 10 to the 23rd power, or 6.02×1023 in mathematical notation. So, in other words, one mole of atoms of carbon-12 is 6.02×1023 carbon atoms. A mole of pumpkin spice lattes, to use a seasonally appropriate reference, is 6.02×1023 pumpkin spice lattes. That many lattes weighs a lot more than 12 grams, though. More on that later.

    So thats why Mole Day is on October 23rd 10/23!

    Yes, the number is the reason for the choice of date. Real purists celebrate between 6:02 AM and 6:02 PM. Although some celebrations happen on June 2 or February 6 .

    6.02×1023 is a huge number. Can you put that in context?

    It can be tough to wrap your mind around a number that big. My high school chemistry textbook put it this way: If you had one mole of marbles 6.02×1023 marbles, that is and you spread them all over the entire surface of the Earth, youd produce a layer thats about three miles thick.

    Ive read that they also call that number Avogadros constant. Why?

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    Formula Mass And The Mole Concept

    • Calculate formula masses for covalent and ionic compounds
    • Define the amount unit mole and the related quantity Avogadroâs numberExplain the relation between mass, moles, and numbers of atoms or molecules, and perform calculations deriving these quantities from one another

    Many argue that modern chemical science began when scientists started exploring the quantitative as well as the qualitative aspects of chemistry. For example, Daltonâs atomic theory was an attempt to explain the results of measurements that allowed him to calculate the relative masses of elements combined in various compounds. Understanding the relationship between the masses of atoms and the chemical formulas of compounds allows us to quantitatively describe the composition of substances.

    What Is A Mole And Why Do We Use It

    The mole is a concept that you’ll see everywhere in chemistry, from GCSE through A level. Although this is the case, it can be very easy to lose sight of what it actually is and why we use it, as your teacher may explain it briefly and then never do so again. It has an exact definition, which you probably will only need to learn exactly if you’re an A level student: the mass of substance that contains the same number of particles as there are atoms in exactly 12.000 g of 12C. This is a complicated way of saying that the mole is a standard number of particles across any compound or element. For example, one mole of hydrogen will contain the same number of particles as one mole of lead, despite them weighing very different amounts. The mole is especially useful when writing chemical equations, because it lets us compare the ratio in which particles react rather than ratio of masses. For example: C + O2 = CO2 shows that one mole of carbon reacts with one mole of oxygen to form one mole of carbon dioxide. This is much more helpful than knowing 1 gram of carbon reacts with 2.7 grams of oxygen to form one mole of carbon dioxide. In case you are asked how many particles or ‘fundamental units’ are in a compound, you should be familiar with the Avogadro Constant: 6.02 x 10^23 mol^-1. This means that there are that many particles in one mole of substance! If you multiple the number of moles of substance you have by the Avogadro constant, you’ll find out how many particles you have.

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    Key Takeaways: How The Mole Units Got Its Name

    • The mole is a unit used in chemistry that is equal to Avogadro’s number. It is the number of carbon atoms in 12 grams of the isotope carbon-12.
    • The word mole comes from the word molecule. It is not related in any way to the animal called the mole.
    • The mole is most often used to convert between numbers of atoms and molecules to the gram mass unit.

    Ostwald is responsible for coming up with the term “mole” , although his original unit was defined in terms of the gram. His later writings made it clear he intended this unit to be based on the ideal gas concept. Around 1900, Ostwald wrote,

    “…the molecular weight of a substance, expressed in grams, shall henceforth be called mole Molekulargewicht eines Stoffes soll fortan ein Mol heissen]”

    “That amount of any gas that occupies a volume of 22414 mL in normal conditions is called one mole ”

    Moles even have their own day, aptly named Mole Day.

    Atomic Weight Molecular Weight And Molar Mass


    By setting the mass of one mole of 12C equal to 12 grams and one 12C atom to 12 amu, the scientists made it possible to easily convert between an elements atomic mass and its molar massthe mass of one mole of molecules. In the case of 12C, we can see that the value for its molar mass and atomic mass both equal 12, although the units are different. While atomic mass is measured in amu, molar mass is measured in grams per mole.

    This shared value between molar mass and atomic mass applies to all elements. Going back to 32S, we know that because its molar mass is 31.97 grams per mole, its atomic mass must be 31.97 amu.

    However, as you can see on the periodic table, sulfur is listed as 32.07 amunot 31.97! This is because the periodic table lists atomic weightsthe averages of the atomic mass for each one of the elements stable isotopes. Specifically, the periodic table lists the average atomic weight calculated based on the relative abundance of an elements different isotopes, as shown in the example with sulfur below.

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    Examples Of Understanding Moles In Chemistry

  • Refer to the molecule O2
    • Since the coefficient of the compound is 1, we are working with 1 mole of the molecule O2
    • 1 mole of O2 = 6.02214 x 1023 molecules O2
  • Since the atom O has a subscript 2, this means that there are two moles of O atoms in the molecule
  • 2 moles of O = 2 * 6.02214 x 1023 atoms = 12.04428 x 1023oxygen atoms
  • 2. Lets try analysis of a compound: CH4

    • Since the coefficient is 1, we are working with 1 mole of the compound CH4
    • 1 mole of CH4 = 6.02214 x 1023 molecules CH4
  • Within the compound there is 1 mole of C, since the subscript is 1
  • 1 mole of C = 6.02214 x 1023 atoms C
  • Within the compound there are 4 moles of H, since the subscript is 4
  • 4 moles of H = 4 * 6.02214 x 1023 atoms = 24.08856 x 1023 atoms H
  • 3. Heres one more thats challenging: 3NaCl

    • Since the coefficient of the compound is 3, we are working with 3 moles of the compound NaCl
    • 3 moles of NaCl = 3 * 6.02214 x 1023 molecules NaCl
  • Within the compound there are 3 mol Na
  • 3 moles of NaCl = 3 * 6.02214 x 1023 = 18.0663 x 1023 atoms of Na
  • Within the compound there are 3 mol Cl
  • 3 moles of NaCl = 3 * 6.02214 x 1023 = 18.0663 x 1023 atoms of Cl
  • How Is The Mole Related To The Dozen

    We can relate the mole to the counting unit: Dozen. We buy eggs by the dozen, we buy soda by the dozen, we buy many things by the dozen, and we know that a dozen always contains 12 things. However, as we buy a dozen of this and a dozen of that, the only number that keeps changing is the mass of the items in the dozen. As a result, we can write a mathematical expression similar to the mole one. Lets say if a dozen eggs weigh 10 g, then we can write something like this: 10 g = 1 Dozen = 12 eggs. If we divide through by 1 Dozen, we will get two ratios:

    • 10 g/1Dozen and
    • 12eggs/1Dozen,

    These ratios are the ones we will use to convert from grams of eggs to Dozen of eggs to number of eggs and in reverse order.

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    What Is The Mole Concept

    The mole concept is a convenient method of expressing the amount of a substance. Any measurement can be broken down into two parts the numerical magnitude and the units that the magnitude is expressed in. For example, when the mass of a ball is measured to be 2 kilograms, the magnitude is 2 and the unit is kilogram.

    When dealing with particles at an atomic level, even one gram of a pure element is known to contain a huge number of atoms. This is where the mole concept is widely used. It primarily focuses on the unit known as a mole, which is a count of a very large number of particles.

    Converting From Grams To Moles

    How To Use Moles – Part 2 | Chemical Calculations | Chemistry | FuseSchool

    It is also possible to convert from grams to moles and vice versa using an element or compounds molecular weight. Recall the the molecular weight of a compound is the sum of the molecular weight of its elemental components.

    • When given the moles of a compound, to find the grams, multiply by molecular weight.
    • When given the grams of a compound, to find the moles, divide by molecular weight.

    To find the atoms of a molecule given the grams, it is first necessary to convert to moles using molecular weight, and then to atoms with Avogadros number.

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    Chapter 5 Stoichiometry And The Mole

  • Describe the unit mole.
  • Relate the mole quantity of substance to its mass.
  • So far, we have been talking about chemical substances in terms of individual atoms and molecules. Yet we dont typically deal with substances an atom or a molecule at a time we work with millions, billions, and trillions of atoms and molecules at a time. What we need is a way to deal with macroscopic, rather than microscopic, amounts of matter. We need a unit of amount that relates quantities of substances on a scale that we can interact with.

    Chemistry uses a unit called mole. A mole is a number of things equal to the number of atoms in exactly 12 g of carbon-12. Experimental measurements have determined that this number is very large:

    1 mol = 6.02214179 × 1023 things

    Understand that a mole means a number of things, just like a dozen means a certain number of thingstwelve, in the case of a dozen. But a mole is a much larger number of things. These things can be atoms, or molecules, or eggs however, in chemistry, we usually use the mole to refer to the amounts of atoms or molecules. Although the number of things in a mole is known to eight decimal places, it is usually fine to use only two or three decimal places in calculations. The numerical value of things in a mole is often called Avogadros number , which is also known as the Avogadro constant, after Amadeo Avogadro, an Italian chemist who first proposed its importance.

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