# why do we want to use the concept of moles Why do chemists need the Mole Concept? Just as in baking, we always want to know how much of one ingredient we must add to another in order to make a desired cake. Similarly, chemists always want to know how much of chemical БAБ (БAБ can be any chemical) will react with how much of chemical B (БBБ can be any chemical) in order to make chemical C (БCБ can be any chemical). On top of that chemists also want to know at the molecular level how many molecules from each chemical will react to make chemical C. Yet, the molecular level consists of particles that are invisible to the naked eye. Because these particles are invisible, chemists canБt count or weigh these individual particles on a scale. So, how do chemists get around this problem? To solve this problem, chemists introduced the
mole concept. The mole is the unit for the SI base quantity the Amount of Substance. WhatБs the Amount of Substance? The Amount of substance is a quantity that measures the size of a pile (collection) of particles. These particles can be electrons, atoms, molecules, ions, or formula units. The amount of substance is an measured quantity that has a symbol n, and a base unit of mole, which is often written in a short way as mol. WhatБs the Mole? The mole is the size of a pile of particles ( amount of substance) that contains as many particles (electrons, atoms, molecules, ions, or formula units) as there are atoms in 12 grams of carbon-12 (an isotope of carbon). What amount of substance can have the same number of particles as in 12g of carbon-12? What do we mean by the atomic mass expressed in grams? It means that we take the relative atomic mass values of all the elements on the periodic table and attach grams to it. Once we do that, we can now weigh 1 mole of any element by simply weighing its exact atomic mass in grams. Similarly, since atoms combine to make molecules, we can find the molecular mass of any molecule by simply finding the sum of all the atomic masses in the chemical formula. Why is it that 1 mole of any element has the same number of particles as in 1 mole of carbon-12? They do because of, and recall that it was his hypothesis that helped scientists determine the of the elements. Now, how many particles (atoms, molecules, or ions) are in 1 mole of a chemical substance? There are 6. 02 x 10 particles in 1 mole of any chemical substance. As you can see, this number is so huge that it has a special name called the AvogadroБs number in honor of Avogadro. Mathematically, we can write a relationship between 1 mole, AvogadroБs number, and atomic mass in grams. б For example, we can write that 1 mole of carbon is equal to 12 g of carbon and in this 12 g there are 6. 02 x 10 atoms of carbon. If we translate that to a mathematical expression, we will get; 1 mole = 12 g = 6. 02 x 10 atoms of carbon. If we divide through by 1 mole, we will get two ratios: 12 g /1mole and 6. 02 x 10 atoms of carbon/1mole. The ratio 12 g/1 mole or 12 g/mol (read as grams per mol) has a special name called Molar Mass. So, the molar mass is simply the mass of 1 mol of a substance. As a result, the only number that will keep changing from substance to substance is the mass of 1 mole of substance (see text in bold). Now, if you need to convert some certain grams of carbon to atoms of carbon or atoms of carbon to grams of carbon, the two ratios (conversion factors) we just derived are the ones you will use. Here is a model that shows the relationship between Mass, Mole, and Avogadro s number Relating Mass, Mole, and Avogadro s number 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. LetБs 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, and 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. б Check you understanding ), how many molecules of CO are in the sample? If you have a sample of 5. 0 g of sulfuric acid, how many mol of sulfuric acid are in the sample? Moles confuse me- why are they used? Think of moles as a "chemist's dozen". Just as 12 eggs is a dozen eggs, 6. 02 10 eggs is a mole of eggs. 6. 02 10 molecules of oxygen is a mole of oxygen. The number of grams in a mole is different from substance to substance. If you're like most students, it's this that's confusing you. Picture it this way: a dozen elephants have a different weight than a dozen rabbits- but in each case, you have a dozen animals. Similarly, a mole of oxygen gas has a different weight than a mole of water- but in each case, you have 6. 02 10 molecules. Why use moles? You often want to know how many molecules you have in a sample of a substance. Counting the molecules individually would be completely impractical. Even if you had a way to see the individual molecules, there are just too many, even in a tiny sample. Moles were defined to solve the problem of counting large numbers of molecules. With moles, you count the number of molecules in the sample by weighing it. Learn to convert between moles and molecules, and moles and grams. Using moles to count molecules. A mole of any substance is 6. 02 10 molecules of that substance. You can use this as a conversion factor to turn moles into molecules. For example, to figure out how many molecules there are in 0. 001 moles of water, You can go the other way, too. To find how many moles are in 100 water molecules, Using masses to count moles. A mole of any substance is the molecular weight of that substance in grams. For example, H O has a molecular weight of 18. 0 (16. 0 for the oxygen and 1. 0 for each hydrogen), so the mole-to-gram relation for water is You can use this as a conversion factor to turn grams to moles. For example, to figure out how many moles there are in 100. 0 g of water, and, you can go the other way. To find out how many grams there are in 6. 0 moles of O, Author:

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