mass fractions of copper(II) oxalate hemihydrate

copper(II) oxalate hemihydrate | elemental composition

Find the elemental composition for copper(II) oxalate hemihydrate 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: CuC_2O_4·0.5H_2O 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 C (carbon) | 4 Cu (copper) | 2 H (hydrogen) | 2 O (oxygen) | 9 N_atoms = 4 + 2 + 2 + 9 = 17 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 C (carbon) | 4 | 4/17 Cu (copper) | 2 | 2/17 H (hydrogen) | 2 | 2/17 O (oxygen) | 9 | 9/17 Check: 4/17 + 2/17 + 2/17 + 9/17 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 4 | 4/17 × 100% = 23.5% Cu (copper) | 2 | 2/17 × 100% = 11.8% H (hydrogen) | 2 | 2/17 × 100% = 11.8% O (oxygen) | 9 | 9/17 × 100% = 52.9% 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 C (carbon) | 4 | 23.5% | 12.011 Cu (copper) | 2 | 11.8% | 63.546 H (hydrogen) | 2 | 11.8% | 1.008 O (oxygen) | 9 | 52.9% | 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 C (carbon) | 4 | 23.5% | 12.011 | 4 × 12.011 = 48.044 Cu (copper) | 2 | 11.8% | 63.546 | 2 × 63.546 = 127.092 H (hydrogen) | 2 | 11.8% | 1.008 | 2 × 1.008 = 2.016 O (oxygen) | 9 | 52.9% | 15.999 | 9 × 15.999 = 143.991 m = 48.044 u + 127.092 u + 2.016 u + 143.991 u = 321.143 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 C (carbon) | 4 | 23.5% | 48.044/321.143 Cu (copper) | 2 | 11.8% | 127.092/321.143 H (hydrogen) | 2 | 11.8% | 2.016/321.143 O (oxygen) | 9 | 52.9% | 143.991/321.143 Check: 48.044/321.143 + 127.092/321.143 + 2.016/321.143 + 143.991/321.143 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 4 | 23.5% | 48.044/321.143 × 100% = 14.96% Cu (copper) | 2 | 11.8% | 127.092/321.143 × 100% = 39.57% H (hydrogen) | 2 | 11.8% | 2.016/321.143 × 100% = 0.6278% O (oxygen) | 9 | 52.9% | 143.991/321.143 × 100% = 44.84%

Mass fraction pie chart