mass fractions of copper(II) nitrate

copper(II) nitrate | elemental composition

Find the elemental composition for copper(II) nitrate 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: Cu(NO_3)_2 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 Cu (copper) | 1 N (nitrogen) | 2 O (oxygen) | 6 N_atoms = 1 + 2 + 6 = 9 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 Cu (copper) | 1 | 1/9 N (nitrogen) | 2 | 2/9 O (oxygen) | 6 | 6/9 Check: 1/9 + 2/9 + 6/9 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Cu (copper) | 1 | 1/9 × 100% = 11.1% N (nitrogen) | 2 | 2/9 × 100% = 22.2% O (oxygen) | 6 | 6/9 × 100% = 66.7% 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 Cu (copper) | 1 | 11.1% | 63.546 N (nitrogen) | 2 | 22.2% | 14.007 O (oxygen) | 6 | 66.7% | 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 Cu (copper) | 1 | 11.1% | 63.546 | 1 × 63.546 = 63.546 N (nitrogen) | 2 | 22.2% | 14.007 | 2 × 14.007 = 28.014 O (oxygen) | 6 | 66.7% | 15.999 | 6 × 15.999 = 95.994 m = 63.546 u + 28.014 u + 95.994 u = 187.554 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 Cu (copper) | 1 | 11.1% | 63.546/187.554 N (nitrogen) | 2 | 22.2% | 28.014/187.554 O (oxygen) | 6 | 66.7% | 95.994/187.554 Check: 63.546/187.554 + 28.014/187.554 + 95.994/187.554 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Cu (copper) | 1 | 11.1% | 63.546/187.554 × 100% = 33.88% N (nitrogen) | 2 | 22.2% | 28.014/187.554 × 100% = 14.94% O (oxygen) | 6 | 66.7% | 95.994/187.554 × 100% = 51.18%

Mass fraction pie chart