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The concepts of atomic mass, moles, and empirical formulas in chemistry. It covers the relationship between atomic mass and avogadro's number, the calculation of molar mass, and the difference between empirical and molecular formulas. The document also includes examples and answers for determining the empirical and molecular formulas of various compounds.
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Since we can use the formula of a known compound to calculate the percent composition by mass of the compound, it is not surprising that we can go in the opposite direction—from experimentally determined percent compositions for an unknown compound, we can calculate the formula of the compound.
Mol K 2 CrO 4 = 5.00 g _ 1 mol 194.2 g
= 0.0257 mol K 2 CrO 4
0.0257 mol _ 6.022 10 formula units^23 1 mol
(^) = 1.55 _ 10 22 formula units K 2 CrO 4
Atoms K = 1.55 _ 10^22 formula units _ 2 K atoms 1 formula un
= 3.10 _ 10 22 atoms K
Atoms Cr = 1.55 _ 10^22 formula units 1 Cr atom 1 formula un
= 1.55 _ 10 22 atoms Cr
Atoms O = 1.55 _ 10^22 formula units ^ 4 O atoms 1 formula un
= 6.20 _ 10 22 atoms O
b. AuCl 3 (molar mass = 303.4 g) Mol AuCl 3 = 5.00 g _ 1 mol
= 0.0165 mol AuCl (^3)
0.0165 mol AuCl 3 _ 6.022 10 formula units^23 1 mol
(^) = 9.94 _ 10 21 formula units AuCl 3
Atoms Au = 9.94 _ 10^21 formula units _ 1 Au atom 1 formula un
= 9.94 _ 10 21 atoms Au
Atoms Cl = 9.94 _ 10 21 formula units _ 3 Cl atoms 1 formula un
= 2.98 _ 10 22 atoms Cl
c. SiH 4 (molar mass = 32.12 g) Mol SiH 4 = 5.00 g _ 1 mol
= 0.156 mol SiH 4
0.156 mol SiH 4 _ 6.022 10molecules^23 1 mol
(^) = 9.39 _ 10 22 molecules SiH 4
Atoms Si = 9.39 _ 10 22 molecules _ 1 Si atom 1 molecul
= 9.39 _ 10 22 atoms Si
Atoms H = 9.39 _ 10^22 molecules _ 4 H atom 1 molecul
= 3.76 _ 10 23 atoms H
d. Ca 3 (PO 4 ) 2 (molar mass = 310.18 g) Mol Ca 3 (PO 4 ) 2 = 5.00 g _ 1 mol
= 0.0161 mol Ca 3 (PO 4 ) (^2)
0.0161 mol Ca 3 (PO 4 ) 2 _ 6.022 10 formula units^23 1 mol
(^) = 9.70 _ 10 21 formula units Ca 3 (PO 4 )^2
Atoms Ca = 9.70 _ 10 21 formula units _ 3 Ca atoms 1 formula un
= 2.91 _ 10 22 atoms Ca
Atoms P = 9.70 _ 10 21 formula units _ 2 P atoms 1 formula un
= 1.94 _ 10 22 atoms P
Atoms O = 9.70 _ 10^21 formula units _ 8 O atoms 1 formula un
= 7.76 _ 10 22 atoms O