Potassium cromate fomula
Potassium chromate is an ionic compound built from potassium ions and the chromate polyatomic ion. The formula is \( \mathrm{K_2CrO_4} \).
Ions and charge balance
Potassium ion: \( \mathrm{K^+} \) (charge \(+1\))
Chromate ion: \( \mathrm{CrO_4^{2-}} \) (charge \(-2\))
Electrical neutrality requires total charge \(0\). Two \( \mathrm{K^+} \) ions balance one \( \mathrm{CrO_4^{2-}} \) ion, giving \( \mathrm{K_2CrO_4} \).
\[ 2 \times (+1) + (-2) = 0 \]Percent comosition from the formula
Percent composition by mass (mass percent) follows from the fixed mass ratios in \( \mathrm{K_2CrO_4} \). The relationship uses molar mass (formula mass) and the mass contribution of each element.
Mass percent for an element \(E\) in a compound:
\[ \%E = \frac{n_E \times M_E}{M_{\text{compound}}}\times 100\% \]\(n_E\) is the number of atoms of \(E\) in the formula, \(M_E\) is the atomic molar mass, and \(M_{\text{compound}}\) is the compound molar mass.
Molar mass of \( \mathrm{K_2CrO_4} \)
Standard atomic masses (g/mol) are used: \(M_{\mathrm{K}} = 39.0983\), \(M_{\mathrm{Cr}} = 51.9961\), \(M_{\mathrm{O}} = 15.999\).
| Element | Symbol | Count in \( \mathrm{K_2CrO_4} \) | Atomic mass (g/mol) | Mass contribution (g/mol) |
|---|---|---|---|---|
| Potassium | K | 2 | 39.0983 | \(2 \times 39.0983 = 78.1966\) |
| Chromium | Cr | 1 | 51.9961 | \(1 \times 51.9961 = 51.9961\) |
| Oxygen | O | 4 | 15.999 | \(4 \times 15.999 = 63.996\) |
Total molar mass:
\[ M_{\mathrm{K_2CrO_4}} = (2 \times 39.0983) + (1 \times 51.9961) + (4 \times 15.999) = 78.1966 + 51.9961 + 63.996 = 194.1887\ \mathrm{g/mol} \]Percent composition by mass
The mass percent values come from each element’s molar-mass contribution divided by \(194.1887\ \mathrm{g/mol}\).
Potassium percent by mass
\[ \%\,\mathrm{K} = \frac{78.1966}{194.1887}\times 100\% \approx 40.268\% \approx 40.27\% \]Chromium percent by mass
\[ \%\,\mathrm{Cr} = \frac{51.9961}{194.1887}\times 100\% \approx 26.776\% \approx 26.78\% \]Oxygen percent by mass
\[ \%\,\mathrm{O} = \frac{63.996}{194.1887}\times 100\% \approx 32.956\% \approx 32.96\% \]Sum check: \(40.27\% + 26.78\% + 32.96\% = 100.01\%\) (rounding).
Visualization of mass contribution in \( \mathrm{K_2CrO_4} \)
Rounding and significant figures
Percent composition depends slightly on the atomic masses chosen (rounded values versus more precise values). Reporting percent composition to 2 decimal places is common for classroom calculations, while laboratory reporting often matches the precision of the given atomic masses and measured data.
Final values
Potassium chromate formula: \( \mathrm{K_2CrO_4} \)
Percent comosition (by mass): K ≈ 40.27%, Cr ≈ 26.78%, O ≈ 32.96%