Using NaCl to illustrate, explain atomic mass and molecular mass, and an estimated and true mass of one molecule. Your explanation should include the appropriate measurement unit for each, and why the estimated and true masses are not exactly identical.

Question

What will be an ideal response?

Answer 1

The atomic mass is the actual mass of an atom, expressed in atomic mass units (amu) or daltons (Da), where 1 amu = 1.6 × 10 -27 kg. Because most of the mass of an atom is contributed by the protons and neutrons, each of which is about 1 amu, the mass can be estimated by counting the number of these particles. The number of protons and neutrons is roughly equal, so the atomic number can simply be doubled to estimate the atomic mass. From the atomic number (see the periodic table) the estimated atomic mass of sodium is 22 amu and of chloride is 34 amu. The mass of a molecule (usually expressed in Da instead of amu) of NaCl is 22 + 34 = 56 Da. From the periodic table, the true weight of Cl is actually closer to 36 amu and of Na is 23 amu, so the true mass of NaCl is closer to 59 Da. There are two reasons why the estimated and actual are not the same: the mass of protons and neutrons is actually greater than one, and the number of neutrons may be more or less than double the number of protons.