mass fractions of sodium arsenate

sodium arsenate | elemental composition

Find the elemental composition for sodium arsenate 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: Na_3AsO_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 As (arsenic) | 1 Na (sodium) | 3 O (oxygen) | 4 N_atoms = 1 + 3 + 4 = 8 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 As (arsenic) | 1 | 1/8 Na (sodium) | 3 | 3/8 O (oxygen) | 4 | 4/8 Check: 1/8 + 3/8 + 4/8 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent As (arsenic) | 1 | 1/8 × 100% = 12.5% Na (sodium) | 3 | 3/8 × 100% = 37.5% O (oxygen) | 4 | 4/8 × 100% = 50.0% 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 As (arsenic) | 1 | 12.5% | 74.921595 Na (sodium) | 3 | 37.5% | 22.98976928 O (oxygen) | 4 | 50.0% | 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 As (arsenic) | 1 | 12.5% | 74.921595 | 1 × 74.921595 = 74.921595 Na (sodium) | 3 | 37.5% | 22.98976928 | 3 × 22.98976928 = 68.96930784 O (oxygen) | 4 | 50.0% | 15.999 | 4 × 15.999 = 63.996 m = 74.921595 u + 68.96930784 u + 63.996 u = 207.88690284 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 As (arsenic) | 1 | 12.5% | 74.921595/207.88690284 Na (sodium) | 3 | 37.5% | 68.96930784/207.88690284 O (oxygen) | 4 | 50.0% | 63.996/207.88690284 Check: 74.921595/207.88690284 + 68.96930784/207.88690284 + 63.996/207.88690284 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent As (arsenic) | 1 | 12.5% | 74.921595/207.88690284 × 100% = 36.04% Na (sodium) | 3 | 37.5% | 68.96930784/207.88690284 × 100% = 33.18% O (oxygen) | 4 | 50.0% | 63.996/207.88690284 × 100% = 30.78%

Mass fraction pie chart