Input interpretation
ammonia | relative molecular mass
Result
Find the relative molecular mass, M_r, for ammonia: 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: NH_3 Use the chemical formula to count the number of atoms, N_i, for each element: | N_i H (hydrogen) | 3 N (nitrogen) | 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 H (hydrogen) | 3 | 1.008 N (nitrogen) | 1 | 14.007 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 H (hydrogen) | 3 | 1.008 | 3 × 1.008 = 3.024 N (nitrogen) | 1 | 14.007 | 1 × 14.007 = 14.007 M_r = 3.024 + 14.007 = 17.031
Comparisons
≈ ( 0.088 ≈ 1/11 ) × relative molecular mass of caffeine ( ≈ 194 )
≈ 0.29 × relative molecular mass of sodium chloride ( ≈ 58 )
≈ 0.95 × relative molecular mass of water ( ≈ 18 )
Corresponding quantities
Molar mass M from M = M_uM_r: | 17 g/mol (grams per mole)
Molecular mass m from m = M_rM_u/N_A: | 2.8×10^-23 grams | 2.8×10^-26 kg (kilograms)