element mass fraction of strontium cyanide dihydrate

strontium cyanide dihydrate | elemental composition

Find the elemental composition for strontium cyanide dihydrate 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: Sr(CN)_2·4H_2O Use the chemical formula, Sr(CN)_2·4H_2O, to count the number of atoms, N_i, for each element and find the total number of atoms, N_atoms: | number of atoms C (carbon) | 2 H (hydrogen) | 4 N (nitrogen) | 2 O (oxygen) | 2 Sr (strontium) | 1 N_atoms = 2 + 4 + 2 + 2 + 1 = 11 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 C (carbon) | 2 | 2/11 H (hydrogen) | 4 | 4/11 N (nitrogen) | 2 | 2/11 O (oxygen) | 2 | 2/11 Sr (strontium) | 1 | 1/11 Check: 2/11 + 4/11 + 2/11 + 2/11 + 1/11 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 2 | 2/11 × 100% = 18.2% H (hydrogen) | 4 | 4/11 × 100% = 36.4% N (nitrogen) | 2 | 2/11 × 100% = 18.2% O (oxygen) | 2 | 2/11 × 100% = 18.2% Sr (strontium) | 1 | 1/11 × 100% = 9.09% 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 C (carbon) | 2 | 18.2% | 12.011 H (hydrogen) | 4 | 36.4% | 1.008 N (nitrogen) | 2 | 18.2% | 14.007 O (oxygen) | 2 | 18.2% | 15.999 Sr (strontium) | 1 | 9.09% | 87.62 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 C (carbon) | 2 | 18.2% | 12.011 | 2 × 12.011 = 24.022 H (hydrogen) | 4 | 36.4% | 1.008 | 4 × 1.008 = 4.032 N (nitrogen) | 2 | 18.2% | 14.007 | 2 × 14.007 = 28.014 O (oxygen) | 2 | 18.2% | 15.999 | 2 × 15.999 = 31.998 Sr (strontium) | 1 | 9.09% | 87.62 | 1 × 87.62 = 87.62 m = 24.022 u + 4.032 u + 28.014 u + 31.998 u + 87.62 u = 175.686 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 C (carbon) | 2 | 18.2% | 24.022/175.686 H (hydrogen) | 4 | 36.4% | 4.032/175.686 N (nitrogen) | 2 | 18.2% | 28.014/175.686 O (oxygen) | 2 | 18.2% | 31.998/175.686 Sr (strontium) | 1 | 9.09% | 87.62/175.686 Check: 24.022/175.686 + 4.032/175.686 + 28.014/175.686 + 31.998/175.686 + 87.62/175.686 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 2 | 18.2% | 24.022/175.686 × 100% = 13.67% H (hydrogen) | 4 | 36.4% | 4.032/175.686 × 100% = 2.295% N (nitrogen) | 2 | 18.2% | 28.014/175.686 × 100% = 15.95% O (oxygen) | 2 | 18.2% | 31.998/175.686 × 100% = 18.21% Sr (strontium) | 1 | 9.09% | 87.62/175.686 × 100% = 49.87%

Mass fraction pie chart