The key to these problems is to recognize that you have polyatomic ions in the
molecule.
a) Al(H2PO4)3 – in this molecule, you have 3 of the polyatomic H 2PO4 (dihydrogen
phosphate).
In one dihydrogen phosphate ion, you have 2 atoms of Hydrogen, 1 atom of
Phosphate, and 4 atoms of Oxygen.
Since you have three of these polyatomic ions, you have 6Hydrogen, 3 atoms
of Phosphate, 12 atoms of Oxygen. You also have 1 atom of Aluminum.
b) HOC6H4COOCH3– In this molecule, you have carbon, oxygen, and hydrogen
atoms at several “locations.”
HOC6H4COOCH38 atoms of hydrogen
H O C6H4C O O C H33 atoms of oxygen
H O C6H4C O O C H38 atoms of carbon
c) (BiO)2SO4 – in this molecule, you have two polyatomic ions that contain oxygen.
You will need to count the oxygen from both sources. There are two
polyatomic ions of bismuth oxide, BiO; so you have 2 atoms of Bismuth and
2 atoms of Oxygen.
You also have 1 sulfur atom and 4 atoms of oxygen in the sulfate polyatomic
ion.
In total, you have 2 atoms of bismuth, 1 sulfur atom, and 8 atoms of
oxygen.
, 2
Problem 9
The problem has two steps: 1) determine the number of atoms in one molecule of
(NH4)2HPO4; then, multiply these values by 3 as you have three molecules.
(NH4)2HPO4
Determine the number of nitrogen and hydrogen in the ammonium ion; there
are 2 N’s and 8 H’s
(NH4)2HPO4
Determine the number of hydrogen, phosphate, and oxygen; there is 1H, 1 P,
and 4 O’s
Now, add up the atoms from each half of the molecule; so, you have:
2 N, 9 H, 1 P, and 4 O. Since you have three molecules, you have a total of
6 N, 27 H, 3 P, and 12 O.
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