element mass fraction of phosphorus trisulfide

phosphorus trisulfide | elemental composition

Find the elemental composition for phosphorus trisulfide 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: P_2S_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 P (phosphorus) | 2 S (sulfur) | 3 N_atoms = 2 + 3 = 5 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 P (phosphorus) | 2 | 2/5 S (sulfur) | 3 | 3/5 Check: 2/5 + 3/5 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent P (phosphorus) | 2 | 2/5 × 100% = 40.0% S (sulfur) | 3 | 3/5 × 100% = 60.0% 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 P (phosphorus) | 2 | 40.0% | 30.973761998 S (sulfur) | 3 | 60.0% | 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 P (phosphorus) | 2 | 40.0% | 30.973761998 | 2 × 30.973761998 = 61.947523996 S (sulfur) | 3 | 60.0% | 32.06 | 3 × 32.06 = 96.18 m = 61.947523996 u + 96.18 u = 158.127523996 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 P (phosphorus) | 2 | 40.0% | 61.947523996/158.127523996 S (sulfur) | 3 | 60.0% | 96.18/158.127523996 Check: 61.947523996/158.127523996 + 96.18/158.127523996 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent P (phosphorus) | 2 | 40.0% | 61.947523996/158.127523996 × 100% = 39.18% S (sulfur) | 3 | 60.0% | 96.18/158.127523996 × 100% = 60.82%

Mass fraction pie chart