Then, add the results together and round off to an appropriate number of significant figures. Solution: To calculate the average atomic weight, each isotopic atomic weight is multiplied by its percent abundance ( expressed as a decimal). Mass number isotopic weight percent abundance Study the tutorial below and then look at the tutorial linked just above.Įxample #1: Calculate the average atomic weight for carbon. This problem can also be reversed, as in having to calculate the isotopic abundances when given the atomic weight and isotopic weights. The older symbol (which the ChemTeam grew up with) is amu (sometimes seen as a.m.u.) The unit amu is still in use, but you will see u used more often. If it is not clear from the context that g/mol is the desired answer, go with amu (which means atomic mass unit).īy the way, the most correct symbol for the atomic mass unit is u. The unit associated with the answers to the problems below can be either amu or g/mol, depending on the context of the question. The ChemTeam prefers to use Wikipedia to look up values. These values can be looked up in a standard reference book such as the "Handbook of Chemistry and Physics." The values can also be looked up via many online sources. (b) the percent abundance for each isotope (a) the exact atomic weight for each naturally- occuring stable isotope To do these problems you need some information. That means the percent by mass of each of the elements is:Ħ.ChemTeam: Calculate the average atomic weight from isotopic weights and abundancesĬalculate the average atomic weight when given isotopic weights and abundancesĬalculate the isotopic abundances, given the atomic weight and isotopic weights The percent by mass of each element is given by the combined molar mass of the atoms of the element in 1 mole of H 3PO 4 divided by the molar mass of H 3PO 4, then multiplied by 100 percent. Grams of urea → moles of urea → moles of H → atoms of HĪssume that you have 1 mole of H 3PO 4. We first need to calculate the molar mass of urea: 60.06 g There are eight C atoms, ten H atoms, four N atoms, and two O atoms in caffeine, so the molecular mass is given by:Ĩ(12.01 amu) + 10(1.008 amu) + 4(14.01 amu) + 2(16.00 amu) = 194.20 amu We can find the atomic masses on the periodic table. For each element, we multiply the atomic mass of the element by the number of atoms of that element in the molecule. To calculate molecular mass, we need to sum all the atomic masses in the molecule. Grams of S → moles of S → number of S atoms We need two conversions: first from grams to moles and then from moles to number of particles (atoms). Multiplying the mass of an isotope by its fractional abundance will give the contribution to the average atomic mass of that isotope. How many grams of oxygen are needed to make 550 grams of CO 2? Possible AnswersĮach isotope contributes to the average atomic mass based on its relative abundance. Consider the following reaction: C 6H 12O 6+ 6 O 2 → 6 CO 2 + 6 H 2O. If 849.2 g of NH 3 is treated with 1223 g of CO 2 how much (NH 2) 2CO is formed?ġ0. Consider the following equation: 2 NH 3 + CO 2 → (NH 2) 2CO + H 2O. How many grams of Li are needed to produce 7.79 g of H 2?ĩ. Consider the following reaction: 2 Li + 2 H 2O → 2 LiOH + H 2. If 968 g of C 6H 12O 6 is consumed, what is the mass of CO 2 produced?Ĩ. Consider the reaction: C 6H 12O 6 + 6 O 2 → 6 CO 2+ 6 H 2O. Balance the following equation with smallest whole number coefficients:ħ. Calculate the percent composition by mass of H, P, and O in this compound.Ħ. How many hydrogen atoms are present in 43.8 g of urea ?ĥ. Calculate the molecular mass (in amu) of caffeine (C 8H 10N 4O 2).Ĥ. How many atoms of sulfur are in 25.1 g of S?ģ. Calculate the average atomic mass of copper.Ģ. Copper has two stable isotopes 63 29Cu (atomic mass = 62.93 amu 69.09% natural abundance) and 65 29Cu (atomic mass = 64.9278 amu 30.91 % natural abundance).
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