Chemical formula
Formula and percent composition for potassium chromate follow from its ionic makeup. Potassium chromate is composed of potassium ions (\(\mathrm{K^+}\)) and the chromate ion (\(\mathrm{CrO_4^{2-}}\)). Electrical neutrality requires two \(\mathrm{K^+}\) ions for each \(\mathrm{CrO_4^{2-}}\) ion, giving the formula K2CrO4.
Formula mass (molar mass) and mass contributions
Standard atomic masses (g/mol) provide the basis for percent composition: \(M(\mathrm{K}) \approx 39.098\), \(M(\mathrm{Cr}) \approx 51.996\), \(M(\mathrm{O}) \approx 15.999\). The molar mass of K2CrO4 is:
\[ M\!\left(\mathrm{K_2CrO_4}\right)=2\,M(\mathrm{K})+1\,M(\mathrm{Cr})+4\,M(\mathrm{O}) \]
\[ M\!\left(\mathrm{K_2CrO_4}\right)=2(39.0983)+51.9961+4(15.999)\approx 194.189\ \mathrm{g/mol}. \]
| Element | Count in K2CrO4 | Atomic mass (g/mol) | Contribution (g/mol) |
|---|---|---|---|
| K | 2 | 39.0983 | 78.1966 |
| Cr | 1 | 51.9961 | 51.9961 |
| O | 4 | 15.999 | 63.9960 |
| Total | — | — | 194.1887 |
Percent composition by mass
Mass percent for an element in a compound is the element’s mass contribution divided by the compound’s molar mass:
\[ \%\,\text{element}=\frac{\text{(count)}\cdot M(\text{element})}{M(\text{compound})}\times 100. \]
| Element | Contribution (g/mol) | Percent by mass |
|---|---|---|
| K | 78.1966 | \(\dfrac{78.1966}{194.1887}\times 100 \approx 40.27\%\) |
| Cr | 51.9961 | \(\dfrac{51.9961}{194.1887}\times 100 \approx 26.78\%\) |
| O | 63.9960 | \(\dfrac{63.9960}{194.1887}\times 100 \approx 32.96\%\) |
| Check | — | \(40.27\%+26.78\%+32.96\%\approx 100.01\%\) (rounding) |
Interpretation and common sources of variation
- Atomic masses as periodic-table averages, with small differences among data tables producing slight percent changes.
- Anhydrous potassium chromate as the implied formula unit; hydrates or mixtures represent different compositions.
- Percent composition as an intrinsic property of the pure compound, independent of sample size.