mass fractions of zinc selenate pentahydrate

zinc selenate pentahydrate | elemental composition

Find the elemental composition for zinc selenate pentahydrate 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: ZnSeO_4·5H_2O Use the chemical formula, ZnSeO_4·5H_2O, to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms: | number of atoms H (hydrogen) | 10 O (oxygen) | 9 Se (selenium) | 1 Zn (zinc) | 1 N_atoms = 10 + 9 + 1 + 1 = 21 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 H (hydrogen) | 10 | 10/21 O (oxygen) | 9 | 9/21 Se (selenium) | 1 | 1/21 Zn (zinc) | 1 | 1/21 Check: 10/21 + 9/21 + 1/21 + 1/21 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent H (hydrogen) | 10 | 10/21 × 100% = 47.6% O (oxygen) | 9 | 9/21 × 100% = 42.9% Se (selenium) | 1 | 1/21 × 100% = 4.76% Zn (zinc) | 1 | 1/21 × 100% = 4.76% 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 H (hydrogen) | 10 | 47.6% | 1.008 O (oxygen) | 9 | 42.9% | 15.999 Se (selenium) | 1 | 4.76% | 78.971 Zn (zinc) | 1 | 4.76% | 65.38 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 H (hydrogen) | 10 | 47.6% | 1.008 | 10 × 1.008 = 10.080 O (oxygen) | 9 | 42.9% | 15.999 | 9 × 15.999 = 143.991 Se (selenium) | 1 | 4.76% | 78.971 | 1 × 78.971 = 78.971 Zn (zinc) | 1 | 4.76% | 65.38 | 1 × 65.38 = 65.38 m = 10.080 u + 143.991 u + 78.971 u + 65.38 u = 298.422 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 H (hydrogen) | 10 | 47.6% | 10.080/298.422 O (oxygen) | 9 | 42.9% | 143.991/298.422 Se (selenium) | 1 | 4.76% | 78.971/298.422 Zn (zinc) | 1 | 4.76% | 65.38/298.422 Check: 10.080/298.422 + 143.991/298.422 + 78.971/298.422 + 65.38/298.422 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent H (hydrogen) | 10 | 47.6% | 10.080/298.422 × 100% = 3.378% O (oxygen) | 9 | 42.9% | 143.991/298.422 × 100% = 48.25% Se (selenium) | 1 | 4.76% | 78.971/298.422 × 100% = 26.46% Zn (zinc) | 1 | 4.76% | 65.38/298.422 × 100% = 21.91%

Mass fraction pie chart