Input interpretation
rubidium fluoride | relative molecular mass
Result
Find the relative molecular mass, M_r, for rubidium fluoride: 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: RbF Use the chemical formula to count the number of atoms, N_i, for each element: | N_i F (fluorine) | 1 Rb (rubidium) | 1 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 F (fluorine) | 1 | 18.998403163 Rb (rubidium) | 1 | 85.4678 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 F (fluorine) | 1 | 18.998403163 | 1 × 18.998403163 = 18.998403163 Rb (rubidium) | 1 | 85.4678 | 1 × 85.4678 = 85.4678 M_r = 18.998403163 + 85.4678 = 104.4662
Comparisons
≈ ( 0.14 ≈ 1/7 ) × relative molecular mass of fullerene ( ≈ 721 )
≈ 0.54 × relative molecular mass of caffeine ( ≈ 194 )
≈ 1.8 × relative molecular mass of sodium chloride ( ≈ 58 )
Corresponding quantities
Molar mass M from M = M_uM_r: | 104 g/mol (grams per mole)
Molecular mass m from m = M_rM_u/N_A: | 1.7×10^-22 grams | 1.7×10^-25 kg (kilograms)