mass fractions of L-phenylalanine

L-phenylalanine | elemental composition

Find the elemental composition for L-phenylalanine 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: C_6H_5CH_2CH(NH_2)CO_2H 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) | 9 H (hydrogen) | 11 N (nitrogen) | 1 O (oxygen) | 2 N_atoms = 9 + 11 + 1 + 2 = 23 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) | 9 | 9/23 H (hydrogen) | 11 | 11/23 N (nitrogen) | 1 | 1/23 O (oxygen) | 2 | 2/23 Check: 9/23 + 11/23 + 1/23 + 2/23 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 9 | 9/23 × 100% = 39.1% H (hydrogen) | 11 | 11/23 × 100% = 47.8% N (nitrogen) | 1 | 1/23 × 100% = 4.35% O (oxygen) | 2 | 2/23 × 100% = 8.70% 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) | 9 | 39.1% | 12.011 H (hydrogen) | 11 | 47.8% | 1.008 N (nitrogen) | 1 | 4.35% | 14.007 O (oxygen) | 2 | 8.70% | 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) | 9 | 39.1% | 12.011 | 9 × 12.011 = 108.099 H (hydrogen) | 11 | 47.8% | 1.008 | 11 × 1.008 = 11.088 N (nitrogen) | 1 | 4.35% | 14.007 | 1 × 14.007 = 14.007 O (oxygen) | 2 | 8.70% | 15.999 | 2 × 15.999 = 31.998 m = 108.099 u + 11.088 u + 14.007 u + 31.998 u = 165.192 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) | 9 | 39.1% | 108.099/165.192 H (hydrogen) | 11 | 47.8% | 11.088/165.192 N (nitrogen) | 1 | 4.35% | 14.007/165.192 O (oxygen) | 2 | 8.70% | 31.998/165.192 Check: 108.099/165.192 + 11.088/165.192 + 14.007/165.192 + 31.998/165.192 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 9 | 39.1% | 108.099/165.192 × 100% = 65.44% H (hydrogen) | 11 | 47.8% | 11.088/165.192 × 100% = 6.712% N (nitrogen) | 1 | 4.35% | 14.007/165.192 × 100% = 8.479% O (oxygen) | 2 | 8.70% | 31.998/165.192 × 100% = 19.37%

Mass fraction pie chart