Chemistry Reference
In-Depth Information
7. Determine the molecular formula of a compound with an empirical
formula of CH
3
and a molecular mass of 30 u (atomic mass units).
a) CH
4
b) C
2
H
8
c) C
2
H
6
d) C
5
H
11
8. Determine the empirical formula and the molecular formula for a
compound that contains 92.3% carbon and 7.7% hydrogen by mass
and a molecular mass of 26.0 u (atomic mass units).
a) CH
2
and C
2
H
4
c) CH
3
and C
2
H
6
b) CH
4
and C
2
H
8
d) CH and C
2
H
2
For each of the following molecular formulas, find the proper empiri-
cal formula.
9.
H
2
O
2
___________________
10. C
4
H
12
_____________________
Lesson 5-5: Molecular and Formula Mass
We have already discussed that fact that you can find the atomic masses
for each of the elements on the Periodic Table of Elements. In this lesson,
we will examine the process of taking these individual elemental masses and
determining the mass of a compound. In reality, you have already needed to
do some of these calculations, but now we will cover them formally. When
dealing with molecular compounds we talk about molecular mass. The
molecular mass is the mass of one molecule of a molecular compound. When
we are dealing with an ionic compound, we can't use the term molecular
mass, because ionic compounds are not made up of individual molecules.
For ionic compounds, we use the term formula mass, which is the mass of one
formula unit. The term formula unit refers to the simplest ratio of the cat-
ions to anions that are found in the compound. NaCl, the formula unit of
table salt, for example, indicates that there is one Na
+
ion for every Cl
-
ion.
Although the terms molecular mass and formula mass are different,
and they really do represent different concepts, the method for calculating
each is identical. In fact, you might not even know if the particular com-
pound that you are calculating the mass for is molecular or ionic, but you
can still get the correct value. Don't get thrown off by the language in the
questions that you are asked to answer. If the question asks for the “mo-
lecular mass,” that is simply an indication that you are working with a mo-
lecular compound. If the question refers to “formula mass,” it is simply
noting that the compound is ionic.
