element counts of phosphorus tribromide

phosphorus tribromide | elemental composition

Find the elemental composition for phosphorus tribromide 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: PBr_3 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 Br (bromine) | 3 P (phosphorus) | 1 N_atoms = 3 + 1 = 4 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 Br (bromine) | 3 | 3/4 P (phosphorus) | 1 | 1/4 Check: 3/4 + 1/4 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Br (bromine) | 3 | 3/4 × 100% = 75.0% P (phosphorus) | 1 | 1/4 × 100% = 25.0% 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 Br (bromine) | 3 | 75.0% | 79.904 P (phosphorus) | 1 | 25.0% | 30.973761998 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 Br (bromine) | 3 | 75.0% | 79.904 | 3 × 79.904 = 239.712 P (phosphorus) | 1 | 25.0% | 30.973761998 | 1 × 30.973761998 = 30.973761998 m = 239.712 u + 30.973761998 u = 270.685761998 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 Br (bromine) | 3 | 75.0% | 239.712/270.685761998 P (phosphorus) | 1 | 25.0% | 30.973761998/270.685761998 Check: 239.712/270.685761998 + 30.973761998/270.685761998 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Br (bromine) | 3 | 75.0% | 239.712/270.685761998 × 100% = 88.56% P (phosphorus) | 1 | 25.0% | 30.973761998/270.685761998 × 100% = 11.44%

Mass fraction pie chart