mass fractions of trimethylamine

trimethylamine | elemental composition

Find the elemental composition for trimethylamine 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)_3N 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) | 9 N (nitrogen) | 1 N_atoms = 3 + 9 + 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) | 3 | 3/13 H (hydrogen) | 9 | 9/13 N (nitrogen) | 1 | 1/13 Check: 3/13 + 9/13 + 1/13 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 3 | 3/13 × 100% = 23.1% H (hydrogen) | 9 | 9/13 × 100% = 69.2% N (nitrogen) | 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) | 3 | 23.1% | 12.011 H (hydrogen) | 9 | 69.2% | 1.008 N (nitrogen) | 1 | 7.69% | 14.007 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 | 23.1% | 12.011 | 3 × 12.011 = 36.033 H (hydrogen) | 9 | 69.2% | 1.008 | 9 × 1.008 = 9.072 N (nitrogen) | 1 | 7.69% | 14.007 | 1 × 14.007 = 14.007 m = 36.033 u + 9.072 u + 14.007 u = 59.112 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 | 23.1% | 36.033/59.112 H (hydrogen) | 9 | 69.2% | 9.072/59.112 N (nitrogen) | 1 | 7.69% | 14.007/59.112 Check: 36.033/59.112 + 9.072/59.112 + 14.007/59.112 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 3 | 23.1% | 36.033/59.112 × 100% = 60.96% H (hydrogen) | 9 | 69.2% | 9.072/59.112 × 100% = 15.35% N (nitrogen) | 1 | 7.69% | 14.007/59.112 × 100% = 23.70%

Mass fraction pie chart