Input interpretation
chromium(III) perchlorate | relative molecular mass
Result
Find the relative molecular mass, M_r, for chromium(III) perchlorate: 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: Cr(ClO_4)_3 Use the chemical formula, Cr(ClO_4)_3, to count the number of atoms, N_i, for each element: | N_i Cl (chlorine) | 3 Cr (chromium) | 1 O (oxygen) | 12 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 Cl (chlorine) | 3 | 35.45 Cr (chromium) | 1 | 51.9961 O (oxygen) | 12 | 15.999 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 Cl (chlorine) | 3 | 35.45 | 3 × 35.45 = 106.35 Cr (chromium) | 1 | 51.9961 | 1 × 51.9961 = 51.9961 O (oxygen) | 12 | 15.999 | 12 × 15.999 = 191.988 M_r = 106.35 + 51.9961 + 191.988 = 350.33
Comparisons
≈ 0.49 × relative molecular mass of fullerene ( ≈ 721 )
≈ 1.8 × relative molecular mass of caffeine ( ≈ 194 )
≈ 6 × relative molecular mass of sodium chloride ( ≈ 58 )
Corresponding quantities
Molar mass M from M = M_uM_r: | 350 g/mol (grams per mole)
Molecular mass m from m = M_rM_u/N_A: | 5.8×10^-22 grams | 5.8×10^-25 kg (kilograms)