mass fractions of potassium peroxide

potassium peroxide | elemental composition

Find the elemental composition for potassium peroxide 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: K_2O_2 Use the chemical formula, K_2O_2, to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms: | number of atoms K (potassium) | 2 O (oxygen) | 2 N_atoms = 2 + 2 = 4 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 K (potassium) | 2 | 2/4 O (oxygen) | 2 | 2/4 Check: 2/4 + 2/4 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent K (potassium) | 2 | 2/4 × 100% = 50.0% O (oxygen) | 2 | 2/4 × 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 K (potassium) | 2 | 50.0% | 39.0983 O (oxygen) | 2 | 50.0% | 15.999 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 K (potassium) | 2 | 50.0% | 39.0983 | 2 × 39.0983 = 78.1966 O (oxygen) | 2 | 50.0% | 15.999 | 2 × 15.999 = 31.998 m = 78.1966 u + 31.998 u = 110.1946 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 K (potassium) | 2 | 50.0% | 78.1966/110.1946 O (oxygen) | 2 | 50.0% | 31.998/110.1946 Check: 78.1966/110.1946 + 31.998/110.1946 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent K (potassium) | 2 | 50.0% | 78.1966/110.1946 × 100% = 70.96% O (oxygen) | 2 | 50.0% | 31.998/110.1946 × 100% = 29.04%

Mass fraction pie chart