mass fractions of potassium chromate

potassium chromate | elemental composition

Find the elemental composition for potassium chromate in terms of the atom and mass percents: atom percent = N_i/N_atoms × 100% mass percent = (N_im_i)/m × 100% Plan: • Write the chemical formula and gather atomic masses from the periodic table. • Determine values for N_i, m_i, N_atoms and m using these items. • Finally, compute the percents and check the results. Write the chemical formula: K_2CrO_4 Use the chemical formula to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms, per molecule: | number of atoms Cr (chromium) | 1 K (potassium) | 2 O (oxygen) | 4 N_atoms = 1 + 2 + 4 = 7 Divide each N_i by N_atoms to calculate atom fractions. Then use the property that atom fractions must sum to one to check the work: | number of atoms | atom fraction Cr (chromium) | 1 | 1/7 K (potassium) | 2 | 2/7 O (oxygen) | 4 | 4/7 Check: 1/7 + 2/7 + 4/7 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Cr (chromium) | 1 | 1/7 × 100% = 14.3% K (potassium) | 2 | 2/7 × 100% = 28.6% O (oxygen) | 4 | 4/7 × 100% = 57.1% Look up the atomic mass, m_i, in unified atomic mass units, u, for each element in the periodic table: | number of atoms | atom percent | atomic mass/u Cr (chromium) | 1 | 14.3% | 51.9961 K (potassium) | 2 | 28.6% | 39.0983 O (oxygen) | 4 | 57.1% | 15.999 Multiply N_i by m_i to compute the mass for each element. Then sum those values to compute the molecular mass, m: | number of atoms | atom percent | atomic mass/u | mass/u Cr (chromium) | 1 | 14.3% | 51.9961 | 1 × 51.9961 = 51.9961 K (potassium) | 2 | 28.6% | 39.0983 | 2 × 39.0983 = 78.1966 O (oxygen) | 4 | 57.1% | 15.999 | 4 × 15.999 = 63.996 m = 51.9961 u + 78.1966 u + 63.996 u = 194.1887 u Divide the mass for each element by m to calculate mass fractions. Then use the property that mass fractions must sum to one to check the work: | number of atoms | atom percent | mass fraction Cr (chromium) | 1 | 14.3% | 51.9961/194.1887 K (potassium) | 2 | 28.6% | 78.1966/194.1887 O (oxygen) | 4 | 57.1% | 63.996/194.1887 Check: 51.9961/194.1887 + 78.1966/194.1887 + 63.996/194.1887 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Cr (chromium) | 1 | 14.3% | 51.9961/194.1887 × 100% = 26.78% K (potassium) | 2 | 28.6% | 78.1966/194.1887 × 100% = 40.27% O (oxygen) | 4 | 57.1% | 63.996/194.1887 × 100% = 32.96%

Mass fraction pie chart