mass fractions of hydrogen fluoride

hydrogen fluoride | elemental composition

Find the elemental composition for hydrogen 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: HF 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) | 1 H (hydrogen) | 1 N_atoms = 1 + 1 = 2 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) | 1 | 1/2 H (hydrogen) | 1 | 1/2 Check: 1/2 + 1/2 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent F (fluorine) | 1 | 1/2 × 100% = 50.0% H (hydrogen) | 1 | 1/2 × 100% = 50.0% 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) | 1 | 50.0% | 18.998403163 H (hydrogen) | 1 | 50.0% | 1.008 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) | 1 | 50.0% | 18.998403163 | 1 × 18.998403163 = 18.998403163 H (hydrogen) | 1 | 50.0% | 1.008 | 1 × 1.008 = 1.008 m = 18.998403163 u + 1.008 u = 20.006403163 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) | 1 | 50.0% | 18.998403163/20.006403163 H (hydrogen) | 1 | 50.0% | 1.008/20.006403163 Check: 18.998403163/20.006403163 + 1.008/20.006403163 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent F (fluorine) | 1 | 50.0% | 18.998403163/20.006403163 × 100% = 94.96% H (hydrogen) | 1 | 50.0% | 1.008/20.006403163 × 100% = 5.038%

Mass fraction pie chart