mass fractions of perfluoroeicosane

perfluoroeicosane | elemental composition

Find the elemental composition for perfluoroeicosane 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: CF_3(CF_2)_18CF_3 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) | 20 F (fluorine) | 42 N_atoms = 20 + 42 = 62 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) | 20 | 20/62 F (fluorine) | 42 | 42/62 Check: 20/62 + 42/62 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 20 | 20/62 × 100% = 32.3% F (fluorine) | 42 | 42/62 × 100% = 67.7% 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 C (carbon) | 20 | 32.3% | 12.011 F (fluorine) | 42 | 67.7% | 18.998403163 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) | 20 | 32.3% | 12.011 | 20 × 12.011 = 240.220 F (fluorine) | 42 | 67.7% | 18.998403163 | 42 × 18.998403163 = 797.932932846 m = 240.220 u + 797.932932846 u = 1038.152932846 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) | 20 | 32.3% | 240.220/1038.152932846 F (fluorine) | 42 | 67.7% | 797.932932846/1038.152932846 Check: 240.220/1038.152932846 + 797.932932846/1038.152932846 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 20 | 32.3% | 240.220/1038.152932846 × 100% = 23.14% F (fluorine) | 42 | 67.7% | 797.932932846/1038.152932846 × 100% = 76.86%

Mass fraction pie chart