mass fractions of thiourea

thiourea | elemental composition

Find the elemental composition for thiourea 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: NH_2CSNH_2 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 C (carbon) | 1 H (hydrogen) | 4 N (nitrogen) | 2 S (sulfur) | 1 N_atoms = 1 + 4 + 2 + 1 = 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 C (carbon) | 1 | 1/8 H (hydrogen) | 4 | 4/8 N (nitrogen) | 2 | 2/8 S (sulfur) | 1 | 1/8 Check: 1/8 + 4/8 + 2/8 + 1/8 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 1 | 1/8 × 100% = 12.5% H (hydrogen) | 4 | 4/8 × 100% = 50.0% N (nitrogen) | 2 | 2/8 × 100% = 25.0% S (sulfur) | 1 | 1/8 × 100% = 12.5% 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) | 1 | 12.5% | 12.011 H (hydrogen) | 4 | 50.0% | 1.008 N (nitrogen) | 2 | 25.0% | 14.007 S (sulfur) | 1 | 12.5% | 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 C (carbon) | 1 | 12.5% | 12.011 | 1 × 12.011 = 12.011 H (hydrogen) | 4 | 50.0% | 1.008 | 4 × 1.008 = 4.032 N (nitrogen) | 2 | 25.0% | 14.007 | 2 × 14.007 = 28.014 S (sulfur) | 1 | 12.5% | 32.06 | 1 × 32.06 = 32.06 m = 12.011 u + 4.032 u + 28.014 u + 32.06 u = 76.117 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) | 1 | 12.5% | 12.011/76.117 H (hydrogen) | 4 | 50.0% | 4.032/76.117 N (nitrogen) | 2 | 25.0% | 28.014/76.117 S (sulfur) | 1 | 12.5% | 32.06/76.117 Check: 12.011/76.117 + 4.032/76.117 + 28.014/76.117 + 32.06/76.117 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 1 | 12.5% | 12.011/76.117 × 100% = 15.78% H (hydrogen) | 4 | 50.0% | 4.032/76.117 × 100% = 5.297% N (nitrogen) | 2 | 25.0% | 28.014/76.117 × 100% = 36.80% S (sulfur) | 1 | 12.5% | 32.06/76.117 × 100% = 42.12%

Mass fraction pie chart