Ferric chloride is the classical (older) name for iron(III) chloride, an ionic compound composed of iron cations and chloride anions. The word “ferric” indicates iron in the \(+3\) oxidation state, distinguishing it from “ferrous” compounds where iron is \(+2\).
Step 1: Decode the name into ions and charges
“Chloride” indicates the anion \( \mathrm{Cl^-} \). “Ferric” indicates the cation \( \mathrm{Fe^{3+}} \) (iron(III)).
Naming equivalence: ferric chloride and iron(III) chloride refer to the same compound; the Roman numeral explicitly states the iron oxidation state.
Step 2: Write the formula by charge balance
An electrically neutral ionic formula requires total positive charge to equal total negative charge. One \( \mathrm{Fe^{3+}} \) requires three \( \mathrm{Cl^-} \) ions:
\[ \mathrm{Fe^{3+}} + 3\,\mathrm{Cl^-} \;\longrightarrow\; \mathrm{FeCl_3} \]
The simplest whole-number ratio of ions is therefore \(1:3\), giving the formula \( \mathrm{FeCl_3} \).
Step 3: Confirm the oxidation state of iron
Chlorine in chloride is \( -1 \). For a neutral compound \( \mathrm{FeCl_3} \), the iron oxidation state \(x\) satisfies:
\[ x + 3(-1) = 0 \quad \Rightarrow \quad x = +3 \]
This matches the “ferric” (iron(III)) designation.
Step 4: Dissociation in water (electrolyte view)
In an aqueous-solution model used in general chemistry, ferric chloride is treated as a strong electrolyte that separates into ions:
\[ \mathrm{FeCl_3(s)} \;\longrightarrow\; \mathrm{Fe^{3+}(aq)} + 3\,\mathrm{Cl^-(aq)} \]
The stoichiometric ion ratio matters in solution calculations: 1 mole of \( \mathrm{FeCl_3} \) produces 1 mole of \( \mathrm{Fe^{3+}} \) and 3 moles of \( \mathrm{Cl^-} \).
Step 5: Molar mass and percent composition
Using standard atomic masses \(M(\mathrm{Fe}) = 55.845\ \mathrm{g\cdot mol^{-1}}\) and \(M(\mathrm{Cl}) = 35.45\ \mathrm{g\cdot mol^{-1}}\), the molar mass of ferric chloride is:
\[ M(\mathrm{FeCl_3}) = 55.845 + 3(35.45) = 55.845 + 106.35 = 162.195\ \mathrm{g\cdot mol^{-1}} \]
| Quantity | Result for ferric chloride | How it is obtained |
|---|---|---|
| Common name | Ferric chloride | Classical naming; “ferric” = iron(III) |
| Stock/IUPAC-style name | Iron(III) chloride | Roman numeral gives oxidation state |
| Formula | \(\mathrm{FeCl_3}\) | Charge balance: \(+3\) with \(3 \times (-1)\) |
| Ions in water | \(\mathrm{Fe^{3+}}\) and \(3\,\mathrm{Cl^-}\) | Dissociation stoichiometry |
| Molar mass | \(162.195\ \mathrm{g\cdot mol^{-1}}\) | \(55.845 + 3(35.45)\) |
Percent by mass follows directly from the molar-mass parts:
\[ \%\mathrm{Fe} = \frac{55.845}{162.195}\times 100 = 34.43\% \qquad \%\mathrm{Cl} = \frac{106.35}{162.195}\times 100 = 65.57\% \]
One quick stoichiometry check
If \(n\) moles of ferric chloride dissolve, the moles of chloride ions produced are \(3n\). For example, for \(0.250\ \mathrm{mol}\) of ferric chloride:
\[ n(\mathrm{Cl^-}) = 3(0.250) = 0.750\ \mathrm{mol} \] \[ m(\mathrm{FeCl_3}) = 0.250 \times 162.195 = 40.54875\ \mathrm{g} \]