Chemistry Reference
In-Depth Information
Notice, the subscript “2”, which is found on the outside of the parenthe-
ses, acts as a multiplier for everything that is inside the parenthesis. For this
reason, the formula Ca(OH)
2
shows 2 atoms of both oxygen and hydrogen. If
you're wondering why we round our answer to 74.1 u, you may want to review
the rules for rounding (Lesson 2-4). Because we are adding, our answer may
only contain as many digits past the decimal point as the addend with the
least number of digits past the decimal. In this case, 40.1 and 32.0 are re-
ported to the tenth place, so we must round our answer to the tenth place.
How do we work with formulas that have subscript numbers both in-
side and outside of the parentheses? A subscript that is on the inside of
parentheses acts as multiplier for only the number that precedes it. A sub-
script that is on the outside of parentheses acts as a multiplier for all of the
elements represented by symbols inside.
Example 4
Find the formula mass of carbon dioxide, Ba(NO
3
)
2
.
Ba = 137 u/atom x 1 atom = 137 u
N
2
= 14.0 u/atom x 2 atoms = 28.0 u
O
6
= 16.0 u/atom x 6 atoms =+ 96.0 u
Total = 261 u
Make sure that you can see how we determined the total number of
atoms for each element in the previous example. There is no subscript as-
sociated with the barium, so it is understood that there is only one atom
per formula unit. The nitrogen has no subscript directly following it, but
there is a subscript of “2” on the outside of the parentheses, which acts as a
multiplier for everything within the parentheses. The oxygen has a sub-
script of “3” following it, which means that there are 3 oxygen atoms per
nitrate ion. We must multiply the 3 by the subscript of “2” that is on the
outside of the parentheses, for a total of six oxygen atoms.
Lesson 5-5 Review
1.
The _______________ mass is the mass of one molecule of a
molecular compound.
2.
The _______________ mass is the mass of one formula unit of an
ionic compound.
