Input interpretation
9-phenanthrylmagnesium bromide | relative molecular mass
Result
Find the relative molecular mass, M_r, for 9-phenanthrylmagnesium bromide: 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: C_14H_9BrMg Use the chemical formula, C_14H_9BrMg, to count the number of atoms, N_i, for each element: | N_i Br (bromine) | 1 C (carbon) | 14 H (hydrogen) | 9 Mg (magnesium) | 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 Br (bromine) | 1 | 79.904 C (carbon) | 14 | 12.011 H (hydrogen) | 9 | 1.008 Mg (magnesium) | 1 | 24.305 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 Br (bromine) | 1 | 79.904 | 1 × 79.904 = 79.904 C (carbon) | 14 | 12.011 | 14 × 12.011 = 168.154 H (hydrogen) | 9 | 1.008 | 9 × 1.008 = 9.072 Mg (magnesium) | 1 | 24.305 | 1 × 24.305 = 24.305 M_r = 79.904 + 168.154 + 9.072 + 24.305 = 281.435
Comparisons
≈ 0.39 × relative molecular mass of fullerene ( ≈ 721 )
≈ 1.4 × relative molecular mass of caffeine ( ≈ 194 )
≈ 4.8 × relative molecular mass of sodium chloride ( ≈ 58 )
Corresponding quantities
Molar mass M from M = M_uM_r: | 281 g/mol (grams per mole)
Molecular mass m from m = M_rM_u/N_A: | 4.7×10^-22 grams | 4.7×10^-25 kg (kilograms)