![]() ![]() 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.Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License. We recommend using aĪuthors: Paul Flowers, Klaus Theopold, Richard Langley, William R. Use the information below to generate a citation. Then you must include on every digital page view the following attribution: If you are redistributing all or part of this book in a digital format, Then you must include on every physical page the following attribution: If you are redistributing all or part of this book in a print format, Want to cite, share, or modify this book? This book uses the This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission. If the molecular (or molar) mass of the substance is known, it may be divided by the empirical formula mass to yield the number of empirical formula units per molecule ( n): As the name suggests, an empirical formula mass is the sum of the average atomic masses of all the atoms represented in an empirical formula. Molecular formulas are derived by comparing the compound’s molecular or molar mass to its empirical formula mass. Molar mass can be measured by a number of experimental methods, many of which will be introduced in later chapters of this text. Molecular mass, for example, is often derived from the mass spectrum of the compound (see discussion of this technique in the previous chapter on atoms and molecules). These quantities may be determined experimentally by various measurement techniques. In both cases, it is the mass of 6.02 × 1023 molecules. One mole of carbon dioxide molecules has a mass of 44.01g. Determining the absolute numbers of atoms that compose a single molecule of a covalent compound requires knowledge of both its empirical formula and its molecular mass or molar mass. The molar mass of any compound is the mass in grams of one mole of that compound. Recall that empirical formulas are symbols representing the relative numbers of a compound’s elements. The percent composition of this compound could be represented as follows: For example, consider a gaseous compound composed solely of carbon and hydrogen. The results of these measurements permit the calculation of the compound’s percent composition, defined as the percentage by mass of each element in the compound. When a compound’s formula is unknown, measuring the mass of each of its constituent elements is often the first step in the process of determining the formula experimentally. The elemental makeup of a compound defines its chemical identity, and chemical formulas are the most succinct way of representing this elemental makeup. ![]() But what if the chemical formula of a substance is unknown? In this section, these same principles will be applied to derive the chemical formulas of unknown substances from experimental mass measurements. Given the chemical formula of the substance, one may determine the amount of the substance (moles) from its mass, and vice versa. ![]() The previous section discussed the relationship between the bulk mass of a substance and the number of atoms or molecules it contains (moles). Determine the molecular formula of a compound.Determine the empirical formula of a compound.Compute the percent composition of a compound.By the end of this section, you will be able to: ![]()
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