mass fractions of tert-butyllithium

tert-butyllithium | elemental composition

Find the elemental composition for tert-butyllithium 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)_3CLi 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) | 4 H (hydrogen) | 9 Li (lithium) | 1 N_atoms = 4 + 9 + 1 = 14 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) | 4 | 4/14 H (hydrogen) | 9 | 9/14 Li (lithium) | 1 | 1/14 Check: 4/14 + 9/14 + 1/14 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 4 | 4/14 × 100% = 28.6% H (hydrogen) | 9 | 9/14 × 100% = 64.3% Li (lithium) | 1 | 1/14 × 100% = 7.14% 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) | 4 | 28.6% | 12.011 H (hydrogen) | 9 | 64.3% | 1.008 Li (lithium) | 1 | 7.14% | 6.94 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) | 4 | 28.6% | 12.011 | 4 × 12.011 = 48.044 H (hydrogen) | 9 | 64.3% | 1.008 | 9 × 1.008 = 9.072 Li (lithium) | 1 | 7.14% | 6.94 | 1 × 6.94 = 6.94 m = 48.044 u + 9.072 u + 6.94 u = 64.056 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) | 4 | 28.6% | 48.044/64.056 H (hydrogen) | 9 | 64.3% | 9.072/64.056 Li (lithium) | 1 | 7.14% | 6.94/64.056 Check: 48.044/64.056 + 9.072/64.056 + 6.94/64.056 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 4 | 28.6% | 48.044/64.056 × 100% = 75.00% H (hydrogen) | 9 | 64.3% | 9.072/64.056 × 100% = 14.16% Li (lithium) | 1 | 7.14% | 6.94/64.056 × 100% = 10.8%

Mass fraction pie chart