element tally of lanthanum sulfide

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lanthanum sulfide | elemental composition

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$Find the elemental composition for lanthanum sulfide 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: La_2S_3 Use the chemical formula, La_2S_3, to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms: | number of atoms La (lanthanum) | 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 La (lanthanum) | 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 La (lanthanum) | 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 La (lanthanum) | 2 | 40.0% | 138.90547 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 La (lanthanum) | 2 | 40.0% | 138.90547 | 2 × 138.90547 = 277.81094 S (sulfur) | 3 | 60.0% | 32.06 | 3 × 32.06 = 96.18 m = 277.81094 u + 96.18 u = 373.99094 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 La (lanthanum) | 2 | 40.0% | 277.81094/373.99094 S (sulfur) | 3 | 60.0% | 96.18/373.99094 Check: 277.81094/373.99094 + 96.18/373.99094 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent La (lanthanum) | 2 | 40.0% | 277.81094/373.99094 × 100% = 74.28% S (sulfur) | 3 | 60.0% | 96.18/373.99094 × 100% = 25.72%$

Find the elemental composition for lanthanum sulfide 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: La_2S_3 Use the chemical formula, La_2S_3, to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms: | number of atoms La (lanthanum) | 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 La (lanthanum) | 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 La (lanthanum) | 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 La (lanthanum) | 2 | 40.0% | 138.90547 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 La (lanthanum) | 2 | 40.0% | 138.90547 | 2 × 138.90547 = 277.81094 S (sulfur) | 3 | 60.0% | 32.06 | 3 × 32.06 = 96.18 m = 277.81094 u + 96.18 u = 373.99094 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 La (lanthanum) | 2 | 40.0% | 277.81094/373.99094 S (sulfur) | 3 | 60.0% | 96.18/373.99094 Check: 277.81094/373.99094 + 96.18/373.99094 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent La (lanthanum) | 2 | 40.0% | 277.81094/373.99094 × 100% = 74.28% S (sulfur) | 3 | 60.0% | 96.18/373.99094 × 100% = 25.72%