mass fractions of lithium dodecyl sulfate

lithium dodecyl sulfate | elemental composition

Find the elemental composition for lithium dodecyl sulfate 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_12H_25LiO_4S 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) | 12 H (hydrogen) | 25 Li (lithium) | 1 O (oxygen) | 4 S (sulfur) | 1 N_atoms = 12 + 25 + 1 + 4 + 1 = 43 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) | 12 | 12/43 H (hydrogen) | 25 | 25/43 Li (lithium) | 1 | 1/43 O (oxygen) | 4 | 4/43 S (sulfur) | 1 | 1/43 Check: 12/43 + 25/43 + 1/43 + 4/43 + 1/43 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 12 | 12/43 × 100% = 27.9% H (hydrogen) | 25 | 25/43 × 100% = 58.1% Li (lithium) | 1 | 1/43 × 100% = 2.33% O (oxygen) | 4 | 4/43 × 100% = 9.30% S (sulfur) | 1 | 1/43 × 100% = 2.33% 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) | 12 | 27.9% | 12.011 H (hydrogen) | 25 | 58.1% | 1.008 Li (lithium) | 1 | 2.33% | 6.94 O (oxygen) | 4 | 9.30% | 15.999 S (sulfur) | 1 | 2.33% | 32.06 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) | 12 | 27.9% | 12.011 | 12 × 12.011 = 144.132 H (hydrogen) | 25 | 58.1% | 1.008 | 25 × 1.008 = 25.200 Li (lithium) | 1 | 2.33% | 6.94 | 1 × 6.94 = 6.94 O (oxygen) | 4 | 9.30% | 15.999 | 4 × 15.999 = 63.996 S (sulfur) | 1 | 2.33% | 32.06 | 1 × 32.06 = 32.06 m = 144.132 u + 25.200 u + 6.94 u + 63.996 u + 32.06 u = 272.328 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) | 12 | 27.9% | 144.132/272.328 H (hydrogen) | 25 | 58.1% | 25.200/272.328 Li (lithium) | 1 | 2.33% | 6.94/272.328 O (oxygen) | 4 | 9.30% | 63.996/272.328 S (sulfur) | 1 | 2.33% | 32.06/272.328 Check: 144.132/272.328 + 25.200/272.328 + 6.94/272.328 + 63.996/272.328 + 32.06/272.328 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 12 | 27.9% | 144.132/272.328 × 100% = 52.93% H (hydrogen) | 25 | 58.1% | 25.200/272.328 × 100% = 9.254% Li (lithium) | 1 | 2.33% | 6.94/272.328 × 100% = 2.55% O (oxygen) | 4 | 9.30% | 63.996/272.328 × 100% = 23.50% S (sulfur) | 1 | 2.33% | 32.06/272.328 × 100% = 11.77%

Mass fraction pie chart