Chemical formulas from compound names
The wording provide the formula for each compound. corresponds to mapping a compound name to a chemical formula written with correct subscripts. Ionic compounds require overall electrical neutrality, while molecular compounds follow atom-count information encoded by prefixes.
Subscripts belong to the identity of a substance and express atom ratios within a formula unit. Coefficients belong to reaction equations and change amounts, not composition.
Core constraint for ionic formulas
Electrical neutrality fixes the ratio of cations and anions. For ions with charges \(a^+\) and \(b^-\), the simplest neutral combination satisfies \[ x(a^+) + y(b^-) = 0, \] with the smallest whole numbers \(x\) and \(y\).
Polyatomic ions remain grouped in parentheses when more than one is required, such as \(\mathrm{(SO_4)_3}\) in \(\mathrm{Al_2(SO_4)_3}\).
Representative set of common compounds and formulas
Standard nomenclature rules produce a unique formula for each name below (ionic salts, molecular compounds, acids, and hydrates). The same logic extends to any similarly named compound.
| Compound name | Type | Charge or prefix information | Formula | Notes |
|---|---|---|---|---|
| Sodium chloride | Ionic (binary) | \(\mathrm{Na^+}\), \(\mathrm{Cl^-}\) | \(\mathrm{NaCl}\) | 1:1 charge balance |
| Calcium chloride | Ionic (binary) | \(\mathrm{Ca^{2+}}\), \(\mathrm{Cl^-}\) | \(\mathrm{CaCl_2}\) | Two chloride ions per calcium ion |
| Aluminum oxide | Ionic (binary) | \(\mathrm{Al^{3+}}\), \(\mathrm{O^{2-}}\) | \(\mathrm{Al_2O_3}\) | Smallest neutral ratio 2:3 |
| Iron(III) chloride | Ionic (binary, variable oxidation state) | \(\mathrm{Fe^{3+}}\), \(\mathrm{Cl^-}\) | \(\mathrm{FeCl_3}\) | Roman numeral fixes \(\mathrm{Fe}\) charge |
| Copper(II) nitrate | Ionic (polyatomic) | \(\mathrm{Cu^{2+}}\), \(\mathrm{NO_3^-}\) | \(\mathrm{Cu(NO_3)_2}\) | Parentheses for multiple nitrates |
| Aluminum sulfate | Ionic (polyatomic) | \(\mathrm{Al^{3+}}\), \(\mathrm{SO_4^{2-}}\) | \(\mathrm{Al_2(SO_4)_3}\) | Common in solubility and stoichiometry work |
| Ammonium carbonate | Ionic (polyatomic cation) | \(\mathrm{NH_4^+}\), \(\mathrm{CO_3^{2-}}\) | \(\mathrm{(NH_4)_2CO_3}\) | Two ammonium ions per carbonate ion |
| Dinitrogen pentoxide | Molecular (binary) | di- = 2 N, penta- = 5 O | \(\mathrm{N_2O_5}\) | Prefixes encode atom counts |
| Carbon monoxide | Molecular (binary) | mono- = 1 O | \(\mathrm{CO}\) | One oxygen atom |
| Sulfur trioxide | Molecular (binary) | tri- = 3 O | \(\mathrm{SO_3}\) | Common acid anhydride of sulfuric acid |
| Hydrochloric acid (aqueous) | Binary acid | H with halide anion | \(\mathrm{HCl}\) | (aq) implied by “acid” naming context |
| Sulfuric acid | Oxoacid | \(\mathrm{SO_4^{2-}}\) conjugate base | \(\mathrm{H_2SO_4}\) | Two acidic H for a \(2-\) oxyanion |
| Sodium hydroxide | Hydroxide | \(\mathrm{Na^+}\), \(\mathrm{OH^-}\) | \(\mathrm{NaOH}\) | Strong base example |
| Copper(II) sulfate pentahydrate | Hydrate | \(\mathrm{CuSO_4}\cdot 5\mathrm{H_2O}\) | \(\mathrm{CuSO_4\cdot 5H_2O}\) | Water of crystallization written after a dot |
Charge neutrality in ionic compounds
Ionic formulas arise from the smallest whole-number ratio of ions that yields zero net charge. Calcium chloride illustrates a \(2+\) cation balanced by two \(1-\) anions: \[ \mathrm{Ca^{2+} + 2Cl^- \rightarrow CaCl_2}. \] Aluminum sulfate illustrates a \(3+\) cation balanced by a \(2-\) polyatomic anion: \[ 2(\mathrm{Al^{3+}}) + 3(\mathrm{SO_4^{2-}}) \rightarrow \mathrm{Al_2(SO_4)_3}. \]
Prefixes and formulas in molecular compounds
Molecular (covalent) names encode atom counts directly through prefixes. Dinitrogen pentoxide contains 2 nitrogen atoms and 5 oxygen atoms, giving \(\mathrm{N_2O_5}\). Carbon monoxide contains 1 oxygen atom, giving \(\mathrm{CO}\). The identity comes from the named elements and the prefix counts rather than charge balancing.
Acids and hydrates
Binary acids combine \(\mathrm{H}\) with a monatomic anion, such as \(\mathrm{HCl}\) for hydrochloric acid in aqueous solution. Oxoacids connect to oxyanions; sulfuric acid corresponds to the sulfate anion \(\mathrm{SO_4^{2-}}\), giving \(\mathrm{H_2SO_4}\) by matching the \(2-\) charge with two acidic hydrogens.
Hydrates append a fixed number of water molecules with a dot. Copper(II) sulfate pentahydrate is \(\mathrm{CuSO_4\cdot 5H_2O}\), where the water count is part of the crystalline composition.
Consistency checks
Element identity remains unchanged by formula writing. Subscripts change only when the compound itself changes; coefficients belong to reaction balancing.
Charge conservation applies to ionic compounds and ionic equations. Molecular formulas apply to neutral molecules, with prefixes fixing atom counts when present in the name.