mass fractions of sodium formate

sodium formate | elemental composition

Find the elemental composition for sodium formate 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: HCOONa 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) | 1 H (hydrogen) | 1 Na (sodium) | 1 O (oxygen) | 2 N_atoms = 1 + 1 + 1 + 2 = 5 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) | 1 | 1/5 H (hydrogen) | 1 | 1/5 Na (sodium) | 1 | 1/5 O (oxygen) | 2 | 2/5 Check: 1/5 + 1/5 + 1/5 + 2/5 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 1 | 1/5 × 100% = 20.0% H (hydrogen) | 1 | 1/5 × 100% = 20.0% Na (sodium) | 1 | 1/5 × 100% = 20.0% O (oxygen) | 2 | 2/5 × 100% = 40.0% 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 C (carbon) | 1 | 20.0% | 12.011 H (hydrogen) | 1 | 20.0% | 1.008 Na (sodium) | 1 | 20.0% | 22.98976928 O (oxygen) | 2 | 40.0% | 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 C (carbon) | 1 | 20.0% | 12.011 | 1 × 12.011 = 12.011 H (hydrogen) | 1 | 20.0% | 1.008 | 1 × 1.008 = 1.008 Na (sodium) | 1 | 20.0% | 22.98976928 | 1 × 22.98976928 = 22.98976928 O (oxygen) | 2 | 40.0% | 15.999 | 2 × 15.999 = 31.998 m = 12.011 u + 1.008 u + 22.98976928 u + 31.998 u = 68.00676928 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) | 1 | 20.0% | 12.011/68.00676928 H (hydrogen) | 1 | 20.0% | 1.008/68.00676928 Na (sodium) | 1 | 20.0% | 22.98976928/68.00676928 O (oxygen) | 2 | 40.0% | 31.998/68.00676928 Check: 12.011/68.00676928 + 1.008/68.00676928 + 22.98976928/68.00676928 + 31.998/68.00676928 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 1 | 20.0% | 12.011/68.00676928 × 100% = 17.66% H (hydrogen) | 1 | 20.0% | 1.008/68.00676928 × 100% = 1.482% Na (sodium) | 1 | 20.0% | 22.98976928/68.00676928 × 100% = 33.81% O (oxygen) | 2 | 40.0% | 31.998/68.00676928 × 100% = 47.05%

Mass fraction pie chart