mass fractions of potassium dithionite

potassium dithionite | elemental composition

Find the elemental composition for potassium dithionite 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_2S_2O_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 K (potassium) | 2 O (oxygen) | 4 S (sulfur) | 2 N_atoms = 2 + 4 + 2 = 8 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 | 2/8 O (oxygen) | 4 | 4/8 S (sulfur) | 2 | 2/8 Check: 2/8 + 4/8 + 2/8 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent K (potassium) | 2 | 2/8 × 100% = 25.0% O (oxygen) | 4 | 4/8 × 100% = 50.0% S (sulfur) | 2 | 2/8 × 100% = 25.0% 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 | 25.0% | 39.0983 O (oxygen) | 4 | 50.0% | 15.999 S (sulfur) | 2 | 25.0% | 32.06 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 | 25.0% | 39.0983 | 2 × 39.0983 = 78.1966 O (oxygen) | 4 | 50.0% | 15.999 | 4 × 15.999 = 63.996 S (sulfur) | 2 | 25.0% | 32.06 | 2 × 32.06 = 64.12 m = 78.1966 u + 63.996 u + 64.12 u = 206.3126 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 | 25.0% | 78.1966/206.3126 O (oxygen) | 4 | 50.0% | 63.996/206.3126 S (sulfur) | 2 | 25.0% | 64.12/206.3126 Check: 78.1966/206.3126 + 63.996/206.3126 + 64.12/206.3126 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent K (potassium) | 2 | 25.0% | 78.1966/206.3126 × 100% = 37.90% O (oxygen) | 4 | 50.0% | 63.996/206.3126 × 100% = 31.02% S (sulfur) | 2 | 25.0% | 64.12/206.3126 × 100% = 31.08%

Mass fraction pie chart