mass fractions of 1-butanol

1-butanol | elemental composition

Find the elemental composition for 1-butanol 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: CH_3(CH_2)_3OH 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) | 4 H (hydrogen) | 10 O (oxygen) | 1 N_atoms = 4 + 10 + 1 = 15 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) | 4 | 4/15 H (hydrogen) | 10 | 10/15 O (oxygen) | 1 | 1/15 Check: 4/15 + 10/15 + 1/15 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 4 | 4/15 × 100% = 26.7% H (hydrogen) | 10 | 10/15 × 100% = 66.7% O (oxygen) | 1 | 1/15 × 100% = 6.67% 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) | 4 | 26.7% | 12.011 H (hydrogen) | 10 | 66.7% | 1.008 O (oxygen) | 1 | 6.67% | 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) | 4 | 26.7% | 12.011 | 4 × 12.011 = 48.044 H (hydrogen) | 10 | 66.7% | 1.008 | 10 × 1.008 = 10.080 O (oxygen) | 1 | 6.67% | 15.999 | 1 × 15.999 = 15.999 m = 48.044 u + 10.080 u + 15.999 u = 74.123 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) | 4 | 26.7% | 48.044/74.123 H (hydrogen) | 10 | 66.7% | 10.080/74.123 O (oxygen) | 1 | 6.67% | 15.999/74.123 Check: 48.044/74.123 + 10.080/74.123 + 15.999/74.123 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 4 | 26.7% | 48.044/74.123 × 100% = 64.82% H (hydrogen) | 10 | 66.7% | 10.080/74.123 × 100% = 13.60% O (oxygen) | 1 | 6.67% | 15.999/74.123 × 100% = 21.58%

Mass fraction pie chart