mass fractions of phenol

phenol | elemental composition

Find the elemental composition for phenol 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_5OH 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) | 6 H (hydrogen) | 6 O (oxygen) | 1 N_atoms = 6 + 6 + 1 = 13 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) | 6 | 6/13 H (hydrogen) | 6 | 6/13 O (oxygen) | 1 | 1/13 Check: 6/13 + 6/13 + 1/13 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 6 | 6/13 × 100% = 46.2% H (hydrogen) | 6 | 6/13 × 100% = 46.2% O (oxygen) | 1 | 1/13 × 100% = 7.69% 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) | 6 | 46.2% | 12.011 H (hydrogen) | 6 | 46.2% | 1.008 O (oxygen) | 1 | 7.69% | 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) | 6 | 46.2% | 12.011 | 6 × 12.011 = 72.066 H (hydrogen) | 6 | 46.2% | 1.008 | 6 × 1.008 = 6.048 O (oxygen) | 1 | 7.69% | 15.999 | 1 × 15.999 = 15.999 m = 72.066 u + 6.048 u + 15.999 u = 94.113 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) | 6 | 46.2% | 72.066/94.113 H (hydrogen) | 6 | 46.2% | 6.048/94.113 O (oxygen) | 1 | 7.69% | 15.999/94.113 Check: 72.066/94.113 + 6.048/94.113 + 15.999/94.113 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 6 | 46.2% | 72.066/94.113 × 100% = 76.57% H (hydrogen) | 6 | 46.2% | 6.048/94.113 × 100% = 6.426% O (oxygen) | 1 | 7.69% | 15.999/94.113 × 100% = 17.00%

Mass fraction pie chart