element tally of chlorine pentafluoride

chlorine pentafluoride | elemental composition

Find the elemental composition for chlorine pentafluoride 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: ClF_5 Use the chemical formula, ClF_5, to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms: | number of atoms Cl (chlorine) | 1 F (fluorine) | 5 N_atoms = 1 + 5 = 6 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 Cl (chlorine) | 1 | 1/6 F (fluorine) | 5 | 5/6 Check: 1/6 + 5/6 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Cl (chlorine) | 1 | 1/6 × 100% = 16.7% F (fluorine) | 5 | 5/6 × 100% = 83.3% 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 Cl (chlorine) | 1 | 16.7% | 35.45 F (fluorine) | 5 | 83.3% | 18.998403163 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 Cl (chlorine) | 1 | 16.7% | 35.45 | 1 × 35.45 = 35.45 F (fluorine) | 5 | 83.3% | 18.998403163 | 5 × 18.998403163 = 94.992015815 m = 35.45 u + 94.992015815 u = 130.442015815 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 Cl (chlorine) | 1 | 16.7% | 35.45/130.442015815 F (fluorine) | 5 | 83.3% | 94.992015815/130.442015815 Check: 35.45/130.442015815 + 94.992015815/130.442015815 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Cl (chlorine) | 1 | 16.7% | 35.45/130.442015815 × 100% = 27.18% F (fluorine) | 5 | 83.3% | 94.992015815/130.442015815 × 100% = 72.82%

Mass fraction pie chart