mass fractions of uranium diselenide

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uranium diselenide | elemental composition

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$Find the elemental composition for uranium diselenide 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: Se_2U Use the chemical formula, Se_2U, to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms: | number of atoms Se (selenium) | 2 U (uranium) | 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 Se (selenium) | 2 | 2/3 U (uranium) | 1 | 1/3 Check: 2/3 + 1/3 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Se (selenium) | 2 | 2/3 × 100% = 66.7% U (uranium) | 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 Se (selenium) | 2 | 66.7% | 78.971 U (uranium) | 1 | 33.3% | 238.02891 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 Se (selenium) | 2 | 66.7% | 78.971 | 2 × 78.971 = 157.942 U (uranium) | 1 | 33.3% | 238.02891 | 1 × 238.02891 = 238.02891 m = 157.942 u + 238.02891 u = 395.97091 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 Se (selenium) | 2 | 66.7% | 157.942/395.97091 U (uranium) | 1 | 33.3% | 238.02891/395.97091 Check: 157.942/395.97091 + 238.02891/395.97091 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Se (selenium) | 2 | 66.7% | 157.942/395.97091 × 100% = 39.89% U (uranium) | 1 | 33.3% | 238.02891/395.97091 × 100% = 60.11%$

Find the elemental composition for uranium diselenide 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: Se_2U Use the chemical formula, Se_2U, to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms: | number of atoms Se (selenium) | 2 U (uranium) | 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 Se (selenium) | 2 | 2/3 U (uranium) | 1 | 1/3 Check: 2/3 + 1/3 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent Se (selenium) | 2 | 2/3 × 100% = 66.7% U (uranium) | 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 Se (selenium) | 2 | 66.7% | 78.971 U (uranium) | 1 | 33.3% | 238.02891 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 Se (selenium) | 2 | 66.7% | 78.971 | 2 × 78.971 = 157.942 U (uranium) | 1 | 33.3% | 238.02891 | 1 × 238.02891 = 238.02891 m = 157.942 u + 238.02891 u = 395.97091 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 Se (selenium) | 2 | 66.7% | 157.942/395.97091 U (uranium) | 1 | 33.3% | 238.02891/395.97091 Check: 157.942/395.97091 + 238.02891/395.97091 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent Se (selenium) | 2 | 66.7% | 157.942/395.97091 × 100% = 39.89% U (uranium) | 1 | 33.3% | 238.02891/395.97091 × 100% = 60.11%