mass fractions of 2,4,6-trimethylstyrene

2, 4, 6-trimethylstyrene | elemental composition

Find the elemental composition for 2, 4, 6-trimethylstyrene 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)_3C_6H_2CH=CH_2 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) | 11 H (hydrogen) | 14 N_atoms = 11 + 14 = 25 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) | 11 | 11/25 H (hydrogen) | 14 | 14/25 Check: 11/25 + 14/25 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 11 | 11/25 × 100% = 44.0% H (hydrogen) | 14 | 14/25 × 100% = 56.0% 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) | 11 | 44.0% | 12.011 H (hydrogen) | 14 | 56.0% | 1.008 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) | 11 | 44.0% | 12.011 | 11 × 12.011 = 132.121 H (hydrogen) | 14 | 56.0% | 1.008 | 14 × 1.008 = 14.112 m = 132.121 u + 14.112 u = 146.233 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) | 11 | 44.0% | 132.121/146.233 H (hydrogen) | 14 | 56.0% | 14.112/146.233 Check: 132.121/146.233 + 14.112/146.233 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 11 | 44.0% | 132.121/146.233 × 100% = 90.35% H (hydrogen) | 14 | 56.0% | 14.112/146.233 × 100% = 9.650%

Mass fraction pie chart