Question 3.11: Applying Various Rules in Writing Formulas Write the formula...
Applying Various Rules in Writing Formulas
Write the formula of the compound (a) tetranitrogen tetrasulfide; (b) ammonium chromate; (c) bromic acid; (d) calcium hypochlorite.
Analyze
We must apply our knowledge of prefixes (such as tetra-) and endings (such as –ic), as well as the names of common polyatomic ions (such as ammonium, chromate, and hypochlorite).
(a) Molecules of this compound consist of four N atoms and four S atoms. The formula is N_{4}S_{4}.
(b) Two ammonium ions ({NH_{4}}^{+}) must be present for every chromate ion ({CrO_{4}}^{2-}). Place parentheses around {NH_{4}}^{+}, followed by the subscript 2. The formula is {(NH_{4})}_{2}CrO_{4}. (This formula is read as “N-H-4, taken twice, C-R-O-4.”)
(c) The –ic acid for the oxoacids of the halogens (group 17) has the halogen in the oxidation state of +5. Bromic acid is HBrO_{3} (analogous to HClO_{3} in Table 3.6).
| TABLE 3.6 Nomenclature of Some Oxoacids and Their Salts | ||||
| Oxidation State | Formula of Acid^{a} | Name of Acid^{b} | Formula of Salt^{b} | Name of Salt |
| Cl: +1 | HClO | Hypochlorous acid | NaClO | Sodium hypochlorite |
| Cl: +3 | HClO_{2} | Chlorous acid | NaClO_{2} | Sodium chlorite |
| Cl: +5 | HClO_{3} | Chloric acid | NaClO_{3} | Sodium chlorate |
| Cl: +7 | HClO_{4} | Perchloric acid | NaClO_{4} | Sodium perchlorate |
| N: +3 | HNO_{2} | Nitrous acid | NaNO_{2} | Sodium nitrite |
| N: +5 | HNO_{3} | Nitric acid | NaNO_{3} | Sodium nitrate |
| S: +4 | H_{2}SO_{3} | Sulfurous acid | {Na}_{2}SO_{3} | Sodium sulfite |
| S: +6 | H_{2}SO_{4} | Sulfuric acid | {Na}_{2}SO_{4} | Sodium sulfate |
_{}^{a}\textrm{In} all these acids, H atoms are bonded to O atoms, not the central nonmetal atom. Often formulas are written to reflect this fact, for instance, HOCl instead of HClO and HOClO instead of HClO_{2}.
_{}^{b}\textrm{In} general, the –ic and –ate names are assigned to compounds in which the central nonmetal atom has an oxidation state equal to the periodic table group number minus 10. Halogen compounds are exceptional in that the -ic and -ate names are assigned to compounds in which the halogen has an oxidation state of +5 (even though the group number is 17).
(d) Here there are one Ca^{2+} and two ClO^{-} ions in a formula unit. This leads to the formula Ca{(ClO)}_{2}.
Assess
Notice that in writing formulas for compounds containing two or more polyatomic ions of the same type, as in (b) and (d), we put parentheses around the formula of the ion (without the charge), followed by a subscript indicating the number of ions of that type. Proper use and placement of parentheses in writing formulas is important.