mass fractions of hexamethylmelamine

hexamethylmelamine | elemental composition

Find the elemental composition for hexamethylmelamine 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_9H_18N_6 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) | 9 H (hydrogen) | 18 N (nitrogen) | 6 N_atoms = 9 + 18 + 6 = 33 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) | 9 | 9/33 H (hydrogen) | 18 | 18/33 N (nitrogen) | 6 | 6/33 Check: 9/33 + 18/33 + 6/33 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 9 | 9/33 × 100% = 27.3% H (hydrogen) | 18 | 18/33 × 100% = 54.5% N (nitrogen) | 6 | 6/33 × 100% = 18.2% 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) | 9 | 27.3% | 12.011 H (hydrogen) | 18 | 54.5% | 1.008 N (nitrogen) | 6 | 18.2% | 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) | 9 | 27.3% | 12.011 | 9 × 12.011 = 108.099 H (hydrogen) | 18 | 54.5% | 1.008 | 18 × 1.008 = 18.144 N (nitrogen) | 6 | 18.2% | 14.007 | 6 × 14.007 = 84.042 m = 108.099 u + 18.144 u + 84.042 u = 210.285 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) | 9 | 27.3% | 108.099/210.285 H (hydrogen) | 18 | 54.5% | 18.144/210.285 N (nitrogen) | 6 | 18.2% | 84.042/210.285 Check: 108.099/210.285 + 18.144/210.285 + 84.042/210.285 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 9 | 27.3% | 108.099/210.285 × 100% = 51.41% H (hydrogen) | 18 | 54.5% | 18.144/210.285 × 100% = 8.628% N (nitrogen) | 6 | 18.2% | 84.042/210.285 × 100% = 39.97%

Mass fraction pie chart