The numerical value of the molar mass of chlorine listed on the periodic table (35.45…
CHEMISTRY · Single correct · A Level · Chemistry · Amount of substance · Amount of substance
Problem
The numerical value of the molar mass of chlorine listed on the periodic table (35.45 g mol−1) is identical to its average atomic mass expressed in atomic mass units (35.45 amu). Which statement best explains why these two numbers are the same?
(a) Both the gram and the atomic mass unit are defined relative to pure 12C, so every element ends up with identical numerical values in each unit.
(b) Periodic table molar masses are rounded so that their gram values match the average atomic masses in amu, purely for calculation convenience.
(c) Because one mole contains 6.022×1023 entities, one atomic mass unit is exactly 1/(6.022×1023) gram, making the mass in grams of a mole of atoms equal numerically to the mass of a single atom in amu.
(d) Natural isotopic abundances cancel the unit difference between grams and amu, causing the two numbers to coincide for elements that have multiple isotopes such as chlorine.