These questions ask you to connect familiar compound names with formulas and to read atom counts or ion ratios from those formulas. Work symbol by symbol so that a small digit, a second appearance of an element or a two-letter symbol is not overlooked.
Turn a formula into a count map
First separate the element symbols. Each symbol starts with a capital letter and may have one following lower-case letter, so Mg is one magnesium symbol rather than M followed by g. Next attach each following digit only to the symbol immediately before it. If there is no digit, record one. Finally, add counts when the same element appears in more than one part of the formula. For example, N2O3 maps to two nitrogen atoms and three oxygen atoms, while C3H7OH contains eight hydrogen atoms because seven appear after H and one appears in OH.
| Familiar substance | Formula pattern to retain |
|---|---|
| Water, hydrogen peroxide, ammonia and methane | H2O, H2O2, NH3 and CH4 respectively |
| Carbon monoxide and carbon dioxide | CO has one oxygen; CO2 has two |
| Sodium chloride and magnesium oxide | NaCl and MgO each show a one-to-one formula-unit ratio |
| Calcium carbonate and sodium hydrogen carbonate | CaCO3 and NaHCO3 respectively |
| Silicon dioxide and ethanol | SiO2 and C2H5OH respectively |
Use molecule for substances made of discrete molecules, but use formula unit and ion ratio for an ionic solid such as sodium chloride. A formula is not repaired by preserving only a ratio: the established formula and the actual counts both matter.
Where formula reading goes wrong
| Trap | How to reject it |
|---|---|
| Dropping a required digit because both element symbols are present | The symbols alone do not preserve the composition. A symbol without a following digit contributes one atom, so CO contains fewer oxygen atoms than CO2. |
| Moving a digit to a neighbouring symbol | Read left to right and attach the digit locally. Changing CaCO3 to Ca2CO3 alters the calcium count; it does not merely rewrite the same composition. |
| Choosing a look-alike symbol | Check capital and lower-case letters before counting. Si is silicon rather than sulfur, and Mg is magnesium rather than manganese. |
| Calling every written composition a molecule | An ionic solid such as sodium chloride is described by a formula unit and an ion ratio. Molecule language wrongly suggests a discrete NaCl molecule. |
| Replacing one familiar compound with a related formula | Shared elements do not make substances interchangeable. Ammonia is NH3 rather than NH4, and hydrogen peroxide retains two hydrogens and two oxygens. |
| Stopping the count after the first group | Scan the entire formula for every occurrence of the element. In C2H5OH, the hydrogen in OH belongs in the total as well as the five beside H. |
Try it
A formula reference gives water as H2O. Each hydrogen nucleus has 1 proton and the oxygen nucleus has 8 protons. Enter the total number of protons represented by one water molecule.
Spot the formula error in a pantry note: it calls sodium hydrogen carbonate Na2CO3. Which formula should replace it?
- NaHCO3
- NaCO3
- Na2CO3
- NaH2CO3
A detector is calibrated for carbon monoxide, whose molecules contain equal numbers of carbon and oxygen atoms. Which formula belongs on its label?
- C2O
- C2O2
- CO2
- CO