The units still cancel appropriately, and we get the same numerical answer in the end. However, it is mathematically equivalent to perform the two calculations sequentially on one line: In this last example, we did the calculation in two steps. So, we will get 1.159 mol of AlCl 3 if we react 123.2 g of Cl 2. Using this conversion factor with the molar quantity we calculated above, we get: The numbers in the conversion factor come from the coefficients in the balanced chemical equation: Now that we have the quantity in moles, we can use the balanced chemical equation to construct a conversion factor that relates the number of moles of Cl 2 to the number of moles of AlCl 3. We must invert this fraction so that the units cancel properly: The molar mass of Cl 2 (which we get from the atomic mass of Cl from the periodic table) is 70.90 g/mol. We know how to do this by simply using the molar mass of Cl 2 as a conversion factor. So to use the balanced chemical equation to relate an amount of Cl 2 to an amount of AlCl 3, we need to convert the given amount of Cl 2 into moles. How can we determine how many moles of AlCl 3 we will get when the reaction is complete? First and foremost, chemical equations are not balanced in terms of grams they are balanced in terms of moles. We can add that conversion factor as another step in a calculation to make a mole-mass calculation, where we start with a given number of moles of a substance and calculate the mass of another substance involved in the chemical equation, or vice versa.įor example, suppose we have the balanced chemical equation: Recall that the molar mass can be determined from a chemical formula and used as a conversion factor. Mole-mole calculations are not the only type of calculations that can be performed using balanced chemical equations. From a given mass of a substance, calculate the mass of another substance involved using the balanced chemical equation.From a given mass of a substance, calculate the moles of another substance involved using the balanced chemical equation.From a given number of moles of a substance, calculate the mass of another substance involved using the balanced chemical equation.Hint: Count the number of atoms of each element, and then multiply that number by the element's atomic weight. Use the table below to find the atomic weight of each atom (element), or refer to a Periodic Table of the Elements. Let us calculate the molecular weight of some common compounds. For example, in one mole of a chemical compound there are 6.022 x 1023 molecules. One mole of 'something' contains 6.022 x 1023 entities. A mole is the unit that measures the amount of a substance. One thousand mers connected together would add up to a weight of 28,000 grams/mole and would have 6,000 atoms.Ī mole is the standard method in chemistry for communicating how much of a substance is present. We combine (react) many mers of ethylene together to form a polyethylene chain. ![]() ![]() The atomic weight of carbon is 12 and that of hydrogen is 1, so one mer of ethylene has a weight of 2(12) + 4(1) = 28. It has a total of 6 atoms: 2 carbon (C) atoms and 4 hydrogen (H) atoms. The chemical formula for an ethylene monomer is -(CH2-CH2).In order to calculate the molecular weight of one water molecule, we add the contributions from each atom that is, 2(1) + 1(16) = 18 grams/mole. Using the periodic table of the elements to find atomic weights, we find that hydrogen has an atomic weight of 1, and oxygen's is 16. The chemical formula for water is H2O, which means this molecule has 3 atoms: 2 of hydrogen (H) and 1 oxygen (O) atom.
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