mass fractions of N-propanol

N-propanol | elemental composition

Find the elemental composition for N-propanol 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_3CH_2CH_2OH 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) | 3 H (hydrogen) | 8 O (oxygen) | 1 N_atoms = 3 + 8 + 1 = 12 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) | 3 | 3/12 H (hydrogen) | 8 | 8/12 O (oxygen) | 1 | 1/12 Check: 3/12 + 8/12 + 1/12 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 3 | 3/12 × 100% = 25.0% H (hydrogen) | 8 | 8/12 × 100% = 66.7% O (oxygen) | 1 | 1/12 × 100% = 8.33% 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) | 3 | 25.0% | 12.011 H (hydrogen) | 8 | 66.7% | 1.008 O (oxygen) | 1 | 8.33% | 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) | 3 | 25.0% | 12.011 | 3 × 12.011 = 36.033 H (hydrogen) | 8 | 66.7% | 1.008 | 8 × 1.008 = 8.064 O (oxygen) | 1 | 8.33% | 15.999 | 1 × 15.999 = 15.999 m = 36.033 u + 8.064 u + 15.999 u = 60.096 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) | 3 | 25.0% | 36.033/60.096 H (hydrogen) | 8 | 66.7% | 8.064/60.096 O (oxygen) | 1 | 8.33% | 15.999/60.096 Check: 36.033/60.096 + 8.064/60.096 + 15.999/60.096 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 3 | 25.0% | 36.033/60.096 × 100% = 59.96% H (hydrogen) | 8 | 66.7% | 8.064/60.096 × 100% = 13.42% O (oxygen) | 1 | 8.33% | 15.999/60.096 × 100% = 26.62%

Mass fraction pie chart