mass
'no magic to the mole'
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
Now try the following question.
2.4 CALCULATING RELATIVE MOLECULAR MASS, Mr, FROM DATA TABLES
Values for relative molecular mass are obtained by multiplying the integer subscript for each type of atom/ion in a formula by its relative atomic mass (Ar value) and summing those results.
N.B. Some prefer the choice of a so-called relative formula mass (also Mr) in preference to the term relative molecular mass, particularly when dealing with ionic compounds, e.g., NaCℓ, MgO, etc.
Realize, however, that relative formula mass is NOT a term recognized by IUPAC and, in any event, values can be referred to simply as Mr, not only for brevity’s sake but also to keep everybody happy.
Now complete the practice exercises below requiring the calculation of Mr values.
The more shortcuts taken when dealing with calculations like those above, e.g., only using a calculator but writing down no working, the more likely it is that errors will arise. While the arithmetic is trivial, generally the more contraction one does, the greater the risk of miscalculation. For this reason, salt hydrates like cobalt(II) chloride hexahydrate are notorious in attracting incorrect relative molecular mass values as candidates' wrong answers to test/exam questions. Where these calculations are anything other than facile, then a tabulated approach is worth considering.
Let’s look again at three compounds we’ve met already.
