mass fractions of lead(II) phthalocyanine

lead(II) phthalocyanine | elemental composition

Find the elemental composition for lead(II) phthalocyanine 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_32H_16N_8Pb Use the chemical formula, C_32H_16N_8Pb, to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms: | number of atoms C (carbon) | 32 H (hydrogen) | 16 N (nitrogen) | 8 Pb (lead) | 1 N_atoms = 32 + 16 + 8 + 1 = 57 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) | 32 | 32/57 H (hydrogen) | 16 | 16/57 N (nitrogen) | 8 | 8/57 Pb (lead) | 1 | 1/57 Check: 32/57 + 16/57 + 8/57 + 1/57 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 32 | 32/57 × 100% = 56.1% H (hydrogen) | 16 | 16/57 × 100% = 28.1% N (nitrogen) | 8 | 8/57 × 100% = 14.0% Pb (lead) | 1 | 1/57 × 100% = 1.75% 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) | 32 | 56.1% | 12.011 H (hydrogen) | 16 | 28.1% | 1.008 N (nitrogen) | 8 | 14.0% | 14.007 Pb (lead) | 1 | 1.75% | 207.2 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) | 32 | 56.1% | 12.011 | 32 × 12.011 = 384.352 H (hydrogen) | 16 | 28.1% | 1.008 | 16 × 1.008 = 16.128 N (nitrogen) | 8 | 14.0% | 14.007 | 8 × 14.007 = 112.056 Pb (lead) | 1 | 1.75% | 207.2 | 1 × 207.2 = 207.2 m = 384.352 u + 16.128 u + 112.056 u + 207.2 u = 719.736 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) | 32 | 56.1% | 384.352/719.736 H (hydrogen) | 16 | 28.1% | 16.128/719.736 N (nitrogen) | 8 | 14.0% | 112.056/719.736 Pb (lead) | 1 | 1.75% | 207.2/719.736 Check: 384.352/719.736 + 16.128/719.736 + 112.056/719.736 + 207.2/719.736 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 32 | 56.1% | 384.352/719.736 × 100% = 53.40% H (hydrogen) | 16 | 28.1% | 16.128/719.736 × 100% = 2.241% N (nitrogen) | 8 | 14.0% | 112.056/719.736 × 100% = 15.57% Pb (lead) | 1 | 1.75% | 207.2/719.736 × 100% = 28.79%

Mass fraction pie chart