Input interpretation
potassium thiosulfate | relative molecular mass
Result
Find the relative molecular mass, M_r, for potassium thiosulfate: M_r = sum _iN_im_i/m_u Plan: • Write the chemical formula and gather atomic masses from the periodic table. • Determine values for N_i and m_i using these items. • Finally, compute the mass. Write the chemical formula: K_2S_2O_3 Use the chemical formula to count the number of atoms, N_i, for each element: | N_i K (potassium) | 2 O (oxygen) | 3 S (sulfur) | 2 Look up the atomic mass, m_i, in unified atomic mass units, u, for each element in the periodic table. Since m_i is divided by the atomic mass constant, m_u, the result is a unitless relative atomic mass: | N_i | m_i/m_u K (potassium) | 2 | 39.0983 O (oxygen) | 3 | 15.999 S (sulfur) | 2 | 32.06 Multiply N_i by m_i/m_u to compute the relative mass for each element. Then sum those values to compute the relative molecular mass, M_r: Answer: | | | N_i | m_i/m_u | relative mass K (potassium) | 2 | 39.0983 | 2 × 39.0983 = 78.1966 O (oxygen) | 3 | 15.999 | 3 × 15.999 = 47.997 S (sulfur) | 2 | 32.06 | 2 × 32.06 = 64.12 M_r = 78.1966 + 47.997 + 64.12 = 190.31
Comparisons
≈ 0.26 × relative molecular mass of fullerene ( ≈ 721 )
≈ 0.98 × relative molecular mass of caffeine ( ≈ 194 )
≈ 3.3 × relative molecular mass of sodium chloride ( ≈ 58 )
Corresponding quantities
Molar mass M from M = M_uM_r: | 190 g/mol (grams per mole)
Molecular mass m from m = M_rM_u/N_A: | 3.2×10^-22 grams | 3.2×10^-25 kg (kilograms)