mass fractions of lanthanum fluoride

lanthanum fluoride | elemental composition

Find the elemental composition for lanthanum fluoride 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: LaF_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 F (fluorine) | 3 La (lanthanum) | 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 F (fluorine) | 3 | 3/4 La (lanthanum) | 1 | 1/4 Check: 3/4 + 1/4 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent F (fluorine) | 3 | 3/4 × 100% = 75.0% La (lanthanum) | 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 F (fluorine) | 3 | 75.0% | 18.998403163 La (lanthanum) | 1 | 25.0% | 138.90547 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 F (fluorine) | 3 | 75.0% | 18.998403163 | 3 × 18.998403163 = 56.995209489 La (lanthanum) | 1 | 25.0% | 138.90547 | 1 × 138.90547 = 138.90547 m = 56.995209489 u + 138.90547 u = 195.900679489 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 F (fluorine) | 3 | 75.0% | 56.995209489/195.900679489 La (lanthanum) | 1 | 25.0% | 138.90547/195.900679489 Check: 56.995209489/195.900679489 + 138.90547/195.900679489 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent F (fluorine) | 3 | 75.0% | 56.995209489/195.900679489 × 100% = 29.09% La (lanthanum) | 1 | 25.0% | 138.90547/195.900679489 × 100% = 70.91%

Mass fraction pie chart