(H2O)4

(H2O)4 (chemical compound)

molar mass | 72.06 g/mol formula | (H2O)4 empirical formula | H_2O_

Find the elemental composition for (H2O)4 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. 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 H (hydrogen) | 8 O (oxygen) | 4 N_atoms = 8 + 4 = 12 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) | 8 | 8/12 O (oxygen) | 4 | 4/12 Check: 8/12 + 4/12 = 1 Compute atom percents using the atom fractions: | number of atoms | atom percent H (hydrogen) | 8 | 8/12 × 100% = 66.7% O (oxygen) | 4 | 4/12 × 100% = 33.3% 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) | 8 | 66.7% | 1.008 O (oxygen) | 4 | 33.3% | 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 H (hydrogen) | 8 | 66.7% | 1.008 | 8 × 1.008 = 8.064 O (oxygen) | 4 | 33.3% | 15.999 | 4 × 15.999 = 63.996 m = 8.064 u + 63.996 u = 72.060 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) | 8 | 66.7% | 8.064/72.060 O (oxygen) | 4 | 33.3% | 63.996/72.060 Check: 8.064/72.060 + 63.996/72.060 = 1 Compute mass percents using the mass fractions: Answer: | | | number of atoms | atom percent | mass percent H (hydrogen) | 8 | 66.7% | 8.064/72.060 × 100% = 11.19% O (oxygen) | 4 | 33.3% | 63.996/72.060 × 100% = 88.81%

Mass fraction pie chart