element tally of dimethylamine

dimethylamine | elemental composition

Find the elemental composition for dimethylamine 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)_2NH 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) | 2 H (hydrogen) | 7 N (nitrogen) | 1 N_atoms = 2 + 7 + 1 = 10 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) | 2 | 2/10 H (hydrogen) | 7 | 7/10 N (nitrogen) | 1 | 1/10 Check: 2/10 + 7/10 + 1/10 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 2 | 2/10 × 100% = 20.0% H (hydrogen) | 7 | 7/10 × 100% = 70.0% N (nitrogen) | 1 | 1/10 × 100% = 10.00% 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) | 2 | 20.0% | 12.011 H (hydrogen) | 7 | 70.0% | 1.008 N (nitrogen) | 1 | 10.00% | 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) | 2 | 20.0% | 12.011 | 2 × 12.011 = 24.022 H (hydrogen) | 7 | 70.0% | 1.008 | 7 × 1.008 = 7.056 N (nitrogen) | 1 | 10.00% | 14.007 | 1 × 14.007 = 14.007 m = 24.022 u + 7.056 u + 14.007 u = 45.085 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) | 2 | 20.0% | 24.022/45.085 H (hydrogen) | 7 | 70.0% | 7.056/45.085 N (nitrogen) | 1 | 10.00% | 14.007/45.085 Check: 24.022/45.085 + 7.056/45.085 + 14.007/45.085 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 2 | 20.0% | 24.022/45.085 × 100% = 53.28% H (hydrogen) | 7 | 70.0% | 7.056/45.085 × 100% = 15.65% N (nitrogen) | 1 | 10.00% | 14.007/45.085 × 100% = 31.07%

Mass fraction pie chart