mass fractions of 3-methoxy 5 hydroxytoluene

3-methoxy 5 hydroxytoluene | elemental composition

Find the elemental composition for 3-methoxy 5 hydroxytoluene 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: C_8H_10O_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) | 8 H (hydrogen) | 10 O (oxygen) | 2 N_atoms = 8 + 10 + 2 = 20 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) | 8 | 8/20 H (hydrogen) | 10 | 10/20 O (oxygen) | 2 | 2/20 Check: 8/20 + 10/20 + 2/20 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent C (carbon) | 8 | 8/20 × 100% = 40.0% H (hydrogen) | 10 | 10/20 × 100% = 50.0% O (oxygen) | 2 | 2/20 × 100% = 10.00% 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) | 8 | 40.0% | 12.011 H (hydrogen) | 10 | 50.0% | 1.008 O (oxygen) | 2 | 10.00% | 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 C (carbon) | 8 | 40.0% | 12.011 | 8 × 12.011 = 96.088 H (hydrogen) | 10 | 50.0% | 1.008 | 10 × 1.008 = 10.080 O (oxygen) | 2 | 10.00% | 15.999 | 2 × 15.999 = 31.998 m = 96.088 u + 10.080 u + 31.998 u = 138.166 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) | 8 | 40.0% | 96.088/138.166 H (hydrogen) | 10 | 50.0% | 10.080/138.166 O (oxygen) | 2 | 10.00% | 31.998/138.166 Check: 96.088/138.166 + 10.080/138.166 + 31.998/138.166 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent C (carbon) | 8 | 40.0% | 96.088/138.166 × 100% = 69.55% H (hydrogen) | 10 | 50.0% | 10.080/138.166 × 100% = 7.296% O (oxygen) | 2 | 10.00% | 31.998/138.166 × 100% = 23.16%

Mass fraction pie chart