mass fractions of strontium bromide

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strontium bromide | elemental composition

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$Find the elemental composition for strontium bromide 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: SrBr_2 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) | 2 Sr (strontium) | 1 N_atoms = 2 + 1 = 3 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) | 2 | 2/3 Sr (strontium) | 1 | 1/3 Check: 2/3 + 1/3 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Br (bromine) | 2 | 2/3 × 100% = 66.7% Sr (strontium) | 1 | 1/3 × 100% = 33.3% 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 Br (bromine) | 2 | 66.7% | 79.904 Sr (strontium) | 1 | 33.3% | 87.62 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) | 2 | 66.7% | 79.904 | 2 × 79.904 = 159.808 Sr (strontium) | 1 | 33.3% | 87.62 | 1 × 87.62 = 87.62 m = 159.808 u + 87.62 u = 247.428 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) | 2 | 66.7% | 159.808/247.428 Sr (strontium) | 1 | 33.3% | 87.62/247.428 Check: 159.808/247.428 + 87.62/247.428 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Br (bromine) | 2 | 66.7% | 159.808/247.428 × 100% = 64.59% Sr (strontium) | 1 | 33.3% | 87.62/247.428 × 100% = 35.41%$

Find the elemental composition for strontium bromide 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: SrBr_2 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) | 2 Sr (strontium) | 1 N_atoms = 2 + 1 = 3 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) | 2 | 2/3 Sr (strontium) | 1 | 1/3 Check: 2/3 + 1/3 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Br (bromine) | 2 | 2/3 × 100% = 66.7% Sr (strontium) | 1 | 1/3 × 100% = 33.3% 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 Br (bromine) | 2 | 66.7% | 79.904 Sr (strontium) | 1 | 33.3% | 87.62 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) | 2 | 66.7% | 79.904 | 2 × 79.904 = 159.808 Sr (strontium) | 1 | 33.3% | 87.62 | 1 × 87.62 = 87.62 m = 159.808 u + 87.62 u = 247.428 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) | 2 | 66.7% | 159.808/247.428 Sr (strontium) | 1 | 33.3% | 87.62/247.428 Check: 159.808/247.428 + 87.62/247.428 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Br (bromine) | 2 | 66.7% | 159.808/247.428 × 100% = 64.59% Sr (strontium) | 1 | 33.3% | 87.62/247.428 × 100% = 35.41%