C4: Chemical Calculations

Use element symbols and ion charges to construct formulae. An ionic compound is electrically neutral, so total positive and negative charge must balance. A molecular formula gives actual atom numbers; an empirical formula gives the simplest whole-number ratio. State symbols are (s), (l), (g), and (aq).

Balance equations by changing coefficients, never subscripts within correct formulae. Ionic equations remove spectator ions and retain only species that change. Check both atom count and charge. The equation coefficients give mole ratios, not direct mass ratios.

One mole contains the Avogadro number of particles. Relative molecular or formula mass is the sum of relative atomic masses. Convert a known mass to moles, apply the balanced-equation ratio, and convert to the requested quantity. At room temperature and pressure, one mole of gas occupies $\pu{24 dm3}$ under the K324 convention.

A limiting reactant is used up first and controls the maximum product. For empirical formula, convert each element mass or percentage to moles, divide by the smallest value, and scale to whole numbers. Use molar mass to move from empirical to molecular formula when the molecular mass is supplied.

Concentration in $\pu{mol.dm-3}$ equals moles divided by solution volume in $\pu{dm3}$. Convert $\pu{cm3}$ to $\pu{dm3}$ before substituting. In titration, use the measured reacting volume and balanced ratio. Concentration in $\pu{g.dm-3}$ can be converted using molar mass when required.

Percentage yield compares actual product with theoretical product. Percentage purity compares the mass of the desired pure substance with the sample mass. The two percentages answer different questions. A result above $100\%$ signals inconsistent data, an uncorrected impurity, wet product, or arithmetic error.