Chemical formulas and percent composition: write formulas for ionic and molecular compounds using oxidation numbers and Table E, and calculate percent composition by mass using Table T.

What this topic is asking

The Core Curriculum asks you to write correct chemical formulas for compounds, using charges (oxidation numbers) and the polyatomic ions on Table E, and to calculate percent composition by mass using the formula on Table T. Both are common Part A and Part B-2 tasks, and they build directly on the gram-formula mass from the previous page.

Writing chemical formulas

For example, aluminum ($\text{Al}^{3+}$) with oxide ($\text{O}^{2-}$): the charges $3$ and $2$ cross over to give $\text{Al}_2\text{O}_3$, whose total charge is $2(3+) + 3(2-) = 0$. When a compound contains more than one polyatomic ion, enclose the ion in parentheses and place the subscript outside: magnesium nitrate is $\text{Mg}(\text{NO}_3)_2$, not $\text{MgNO}_{32}$. Table E lists the names, formulas and charges of the common polyatomic ions you need.

Percent composition by mass

Table T gives the formula directly:

If an element appears more than once, use its total mass (the atomic mass times its subscript). Percent composition is useful for checking the purity of a sample and is the first step in finding a formula from experimental data.

Multivalent metals and naming

Some metals, especially the transition elements, can form ions of more than one charge, so their compounds are named with a Roman numeral (the Stock system) to show the oxidation number of the metal. Iron(II) chloride is $\text{FeCl}_2$ (the iron is $2+$), while iron(III) chloride is $\text{FeCl}_3$ (the iron is $3+$). When a question names such a compound, the Roman numeral tells you the cation charge to balance against the anion, and the periodic table or Table E supplies the rest. Reading the charge correctly here is the difference between writing $\text{FeCl}_2$ and $\text{FeCl}_3$.

Linking back to the mole

Once you can write a formula and find its gram-formula mass, percent composition follows directly, and so does the empirical-formula reasoning used in stoichiometry. The empirical formula is the simplest whole-number ratio of atoms in a compound; you find it by converting each element's mass (or percent) to moles and dividing by the smallest. A formula is itself a mole statement: $\text{CaCO}_3$ contains one mole of calcium, one of carbon and three of oxygen per mole of compound, which is exactly the information the percent-composition calculation uses. Because the periodic-table atomic masses already account for isotopes, every formula and percent-composition result is grounded in the same gram-formula mass you met on the previous page.

Try this

Q1. Write the formula of the compound formed between $\text{Al}^{3+}$ and $\text{SO}_4^{2-}$ (sulfate, from Table E). [1 point]

• Cue. Charges $3$ and $2$ cross over: $\text{Al}_2(\text{SO}_4)_3$.

Q2. Calculate the percent by mass of hydrogen in water, $\text{H}_2\text{O}$ (gram-formula mass $18.0$). [2 points]

• Cue. $\dfrac{2(1.0)}{18.0} \times 100 = 11.1\%$.