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You'll only have to go to grams IF the number of moles is asked for. Just work the problem in pounds - it will work. there is really no reason to convert to grams first and then back out to pounds later. How? Well IF the problem is stated in say pounds, and then wants the answer in pounds. Quite the rigorous path for "all" problems. And finally, convert those grams into any other unit needed that might be asked for. Then (if need be) convert your answer in moles into grams. Then, convert those grams in to moles and work the problem in moles only. In general, to work all types of stoichiometry problems, we say to convert all masses to grams first. Those atomic weights are the number of grams you will need of that element in order to have exactly 1 mole of that element. Counting by number is the molar amount, while measuring by mass is the. This helps tremendously when having to convert from moles to mass as we often do in chemistry. Because of that old definition, we were able to say that all those atomic weights are in grams per mole of substance or abbreviated g/mol. So why DO we seem to concentrate on the "gram" as our go to guy on the periodic table for atomic weights and ultimately for molar masses and molecular weights? Well the key here is the way we historically defined the mole. All chemical ratios work just as well with masses as they do with our oh so familiar moles. You can work chemistry mass problems in any mass you want and it will still work because the masses are relative to each other. Not to mention the myriad of masses represented by all the metric prefixes to prepend to "gram". short tons, long tons, drams, grains, or stones. You could think in pounds, or kilograms, or ounces, or even tons, or heaven forbid. Relative masses means that they are all corrected relative to each other. BUT it would be much much better for you to realize that those could be ANY unit of weight/mass you choose and the whole table would still be correct. "Well, I know the weights are in grams because that is how I learned it in high school". Notice how the atomic weights have no units after them. Hey you! LOOK again at any periodic table - including the one above. The diagram below illustrates the parts and their definitions. You need to make sure that you know what each of these parts is and what it represents. You want a lot more periodic tables to chose from? *Note: If you click on the table, you'll launch it into its own window/page on your browser. These three pieces of data are the elemental symbol, the atomic number (typically given the symbol, Z, and the atomic weight. In it's simplest form (shown below), each entry only has three pieces of information that you will need to know. (in other words we reduced 100% to decimal form 1.The periodic table can often be presented with an abundance of data about each and every element listed. We will let 6Li = x and 7 Li = 1-x we use 1 – x instead of 100 – x because the small number is easier to work with. Since I don’t know what the percentage are, I will have to use variables.ġ00% of Lithium is determined by these two naturally occurring isotopes. Determine the percent abundance of each isotope.Īw = + + Ħ.94 = + The atomic mass of lithium is 6.94, the naturally occurring isotopes are 6Li = 6.015121 amu, and 7Li = 7.016003 amu. What are the percent abundances of the isotopes? Since the overall atomic weight for copper is not given in the problem, you must look it up in the periodic table to work this solution. If you look in the periodic table you will be able to check that our answer is correct!ģVerify that the atomic mass of magnesium is 24.31, given the followingĪtomic mass= + + ĭetermining the percent abundance of each isotope from atomic mass.Ĭopper exists as two isotopes: 63Cu (62.9298 amu) and 65Cu (64.9278 amu). ![]() 10.81amu so, the atomic weight of B = 10.81amu
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