mass fractions of silicon tetrabromide

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silicon tetrabromide | elemental composition

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$Find the elemental composition for silicon tetrabromide 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: SiBr_4 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) | 4 Si (silicon) | 1 N_atoms = 4 + 1 = 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 Br (bromine) | 4 | 4/5 Si (silicon) | 1 | 1/5 Check: 4/5 + 1/5 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Br (bromine) | 4 | 4/5 × 100% = 80.0% Si (silicon) | 1 | 1/5 × 100% = 20.0% 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) | 4 | 80.0% | 79.904 Si (silicon) | 1 | 20.0% | 28.085 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) | 4 | 80.0% | 79.904 | 4 × 79.904 = 319.616 Si (silicon) | 1 | 20.0% | 28.085 | 1 × 28.085 = 28.085 m = 319.616 u + 28.085 u = 347.701 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) | 4 | 80.0% | 319.616/347.701 Si (silicon) | 1 | 20.0% | 28.085/347.701 Check: 319.616/347.701 + 28.085/347.701 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Br (bromine) | 4 | 80.0% | 319.616/347.701 × 100% = 91.92% Si (silicon) | 1 | 20.0% | 28.085/347.701 × 100% = 8.077%$

Find the elemental composition for silicon tetrabromide 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: SiBr_4 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) | 4 Si (silicon) | 1 N_atoms = 4 + 1 = 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 Br (bromine) | 4 | 4/5 Si (silicon) | 1 | 1/5 Check: 4/5 + 1/5 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Br (bromine) | 4 | 4/5 × 100% = 80.0% Si (silicon) | 1 | 1/5 × 100% = 20.0% 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) | 4 | 80.0% | 79.904 Si (silicon) | 1 | 20.0% | 28.085 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) | 4 | 80.0% | 79.904 | 4 × 79.904 = 319.616 Si (silicon) | 1 | 20.0% | 28.085 | 1 × 28.085 = 28.085 m = 319.616 u + 28.085 u = 347.701 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) | 4 | 80.0% | 319.616/347.701 Si (silicon) | 1 | 20.0% | 28.085/347.701 Check: 319.616/347.701 + 28.085/347.701 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Br (bromine) | 4 | 80.0% | 319.616/347.701 × 100% = 91.92% Si (silicon) | 1 | 20.0% | 28.085/347.701 × 100% = 8.077%