This is usually due to rounding off by the publisher. Also, the atomic weights in your periodic table may be slightly different than the ones in the example. The placement of the atomic weights may be sligtly different in the periodic table you are using. These numbers are the atomic weight for each element. For carbon, it is 12.011 and for oxygen it is 15.9994. The important number to use is at the bottom, just below the element symbol. Here are some typical element tiles, as they might appear in a periodic table: The atomic weight of each element is found by examining the periodic table. Point #2: You need to know the atomic weight of each element in order to calculate the molecular weight of the substance. One copper, one sulfur, 10 hydrogen, nine oxygen. (e) Al 2(SO 4) 3 Two aluminum, three sulfur, and 12 oxygen. Note how nitrogen is in two different places in the formula. (d) NH 4NO 3 Two nitrogen, four H and three O. (c) Al(NO 3) 3 One Al, three nitrogen, and nine oxygens. (b) Fe 2O 3 There are two irons and three oxygens. If parentheses are involved, you must multiply each subscript inside by the one which is outside.Įxample #1: How many of each element are in the following examples? If a subscript follows an atom with no parenthesis, that number tells you how many of that atom are present. Mg(OH) 2 has one atom of magnesiun and two each of oxygen and hydrogen. H 2O 2 has two atoms each of oxygen and hydrogen. Point #1: You need to know how many atoms of each element are in a substance in order to calculate its molecular weight.įor example H 2O has two atoms of hydrogen and one atom of oxygen. There are many Internet sites that allow you to input a formula and get the molecular weight. You should have a periodic table for looking up atomic weights and a calculator. The mole is the standard method in chemistry for communicating how much of a substance is present. The molecular weight of a substance is needed to tell us how many grams are in one mole of that substance. If you weigh twice as much, you weigh 24. For example, if you weigh 1/2 as much as C-12, you weigh 6. This is the starting point for how much an atom weighs. (And, of course, amu still exists in many older documents pre-dating 1961.) Consequently, you need to be able to recognize and use both.ĭefinition #3: One atom of the carbon-12 isotope is defined as weighing exactly 12 amu. While the unit u was intended to replace amu, both continue to be used. The ChemTeam will use both u and amu in problem solutions. The most complete definition is a bit more technical (1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest), but the above definition is sufficient for the introductory level.Īn older symbol for atomic mass unit is amu, while the most recent symbol is u (a lower case letter u) which comes from the term 'unified atomic mass unit.' Unified comes from a 1961 agreement between the chemistry and physics communities to use the same definition for the atomic mass unit. The factor-label method yields the desired cancellation of units, and the computed result is on the order of 10 22 as expected.There are a number of terms you need to know, at the same time, in order to know what the term "molecular weight" means.ĭefinition #1: The molecular weight of a substance is the weight in atomic mass units of all the atoms in a given formula.ĭefinition #2: An atomic mass unit is defined as 1/12 the weight of one atom of the carbon-12 isotope.
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