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# Scan_Doc0090 - 84 Chapter Three Stoichiometry Reality Check...

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84 Chapter Three Stoichiometry EXAMPLE 3.5 ---:== Fragments of cobalt metal. In this case, the term 12.01 limits the number of significant figures. A substance's molar mass is the mass in grams of 1 mole of the substance. Reality Check: Note that 5.68 mg of silicon is clearly much less than 1 mol of silicon (which has a mass of 28.09 g), so the final answer of 1.22 X 10 20 atoms (compared with 6.022 X 10 23 atoms) is in the right direction. SEE EXERCISE 3.43 Cobalt (Co) is a metal that is added to steel to improve its resistance to corrosion. Calcu- late both the number of moles in a sample of cobalt containing 5.00 X 10 20 atoms and the mass of the sample. Solution Note that the sample of 5.00 X 10 20 atoms of cobalt is less than 1 mole (6.022 X 1023 atoms) of cobalt. What fraction of a mole it represents can be determined as follows: 1 mol Co 5.00 X 10 20 ~ X 23 = 8.30 X 10- 4 mol Co 6.022 X 10 ~ Since the mass of 1 mole of cobalt atoms is 58.93 g, the mass of 5.00 X 10 20 atoms can be determined
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