Input interpretation
2, 5-dibromo-3-octylthiophene | relative molecular mass
Result
Find the relative molecular mass, M_r, for 2, 5-dibromo-3-octylthiophene: 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_12H_18Br_2S Use the chemical formula, C_12H_18Br_2S, to count the number of atoms, N_i, for each element: | N_i Br (bromine) | 2 C (carbon) | 12 H (hydrogen) | 18 S (sulfur) | 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) | 2 | 79.904 C (carbon) | 12 | 12.011 H (hydrogen) | 18 | 1.008 S (sulfur) | 1 | 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 Br (bromine) | 2 | 79.904 | 2 × 79.904 = 159.808 C (carbon) | 12 | 12.011 | 12 × 12.011 = 144.132 H (hydrogen) | 18 | 1.008 | 18 × 1.008 = 18.144 S (sulfur) | 1 | 32.06 | 1 × 32.06 = 32.06 M_r = 159.808 + 144.132 + 18.144 + 32.06 = 354.14
Comparisons
≈ 0.49 × relative molecular mass of fullerene ( ≈ 721 )
≈ 1.8 × relative molecular mass of caffeine ( ≈ 194 )
≈ 6.1 × relative molecular mass of sodium chloride ( ≈ 58 )
Corresponding quantities
Molar mass M from M = M_uM_r: | 354 g/mol (grams per mole)
Molecular mass m from m = M_rM_u/N_A: | 5.9×10^-22 grams | 5.9×10^-25 kg (kilograms)