Input interpretation
water | molecular mass
Result
Find the molecular mass, m, for water: m = sum _iN_im_i 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: H_2O Use the chemical formula to count the number of atoms, N_i, for each element: | N_i H (hydrogen) | 2 O (oxygen) | 1 Look up the atomic mass, m_i, in unified atomic mass units, u, for each element in the periodic table: | N_i | m_i/u H (hydrogen) | 2 | 1.008 O (oxygen) | 1 | 15.999 Multiply N_i by m_i to compute the mass for each element. Then sum those values to compute the molecular mass, m: Answer: | | | N_i | m_i/u | mass/u H (hydrogen) | 2 | 1.008 | 2 × 1.008 = 2.016 O (oxygen) | 1 | 15.999 | 1 × 15.999 = 15.999 m = 2.016 u + 15.999 u = 18.015 u
Unit conversions
18.015 Da (daltons)
0.018015 kDa (kilodaltons)
2.9915×10^-23 grams
2.9915×10^-26 kg (kilograms)
18.016 chemical atomic mass units (unit officially deprecated)
18.021 physical atomic mass units (unit officially deprecated)
Comparisons as mass of molecule
≈ ( 0.025 ≈ 1/40 ) × molecular mass of fullerene ( ≈ 721 u )
≈ ( 0.093 ≈ 1/11 ) × molecular mass of caffeine ( ≈ 194 u )
≈ 0.31 × molecular mass of sodium chloride ( ≈ 58 u )
Corresponding quantities
Relative atomic mass A_r from A_r = m_aN_A/M_u: | 18
Molar mass M from M = m_aN_A: | 18 g/mol (grams per mole)
Relative molecular mass M_r from M_r = m_mN_A/M_u: | 18
Molar mass M from M = m_mN_A: | 18 g/mol (grams per mole)