Video Transcript
Which of these substances has the
largest formula mass? The relative atomic mass of carbon
is 12, hydrogen is one, oxygen 16, sodium 23, bromine 80, chlorine 35.5, fluorine
19, and nitrogen 14. (A) The anesthetic halothane,
C2HBrClF3; (B) the herbicide paraquat, C12H14N2Cl2; (C) caffeine, C8H10N4O2; (D)
urea, CO(NH2)2; (E) a typical soap, C17H35CO2Na.
The formula mass is the average
mass of one formula unit of a compound. We can calculate the formula masses
of each compound we were given using the relative atomic masses given in the
problem. The relative atomic mass is the
average mass of one atom of an element relative to the mass of carbon-12, which is
defined to be 12. In reality, the mass of the
carbon-12 atom is about 1.99 times 10 to the negative 23 grams. But that’s not a very convenient
number to work with in calculations, which is why we use relative atomic masses.
Let’s calculate the formula masses
of each compound given in the answer choices, starting with the anesthetic
halothane. We can determine which elements are
present in halothane by looking at the chemical formula. Halothane contains the elements
carbon, hydrogen, bromine, chlorine, and fluorine. The relative atomic masses of each
element were listed in the problem. We can look at the subscripts in
the chemical formula to determine how many atoms of each element are in one formula
unit. There are two carbon atoms, one
hydrogen atom, one bromine atom, one chlorine atom, and three fluorine atoms in one
formula unit of halothane.
To calculate the formula mass, we
just multiply the mass of each element by the number of atoms in one formula unit
and add the results. Doing that calculation gives us
197.5. So, let’s take note of that and
move on to the next molecule, the herbicide paraquat.
Paraquat contains the elements
carbon, hydrogen, nitrogen, and chlorine. We’ll note that and the relative
atomic masses of each element. There are 12 carbon atoms, 14
hydrogen atoms, two nitrogen atoms, and two chlorine atoms in one formula unit of
paraquat. If we multiply and add all those
values, we’ll find the formula mass of paraquat is 257. So, let’s note that formula mass as
well and move on to caffeine.
We can peek at the chemical formula
to figure out which elements are in caffeine. Then, we’ll note the relative
atomic masses of each. Now, we’ll multiply those atomic
masses by the number of atoms of each element in caffeine. That calculation gives us 194 for
the formula mass.
Now we can calculate the mass of
urea, answer choice (D). Urea contains carbon, oxygen,
nitrogen, and hydrogen. Let’s note the number of atoms
indicated by the chemical formula in urea. Remember the subscript two outside
the parentheses indicates that there are two units of NH2 in urea. So, there are a total of two
nitrogen and four hydrogen atoms in one formula unit of urea. Multiplying then adding all those
together gives us 60 as the formula mass of urea.
Now, we can move on to the final
substance we were given, a typical soap. A typical soap contains carbon,
hydrogen, oxygen, and sodium. Looking at the number of each atom
in a formula unit, note that carbon appears in two places in the formula. So, there are a total of 18 carbon
atoms in a formula unit. If we do the math, we’ll find the
formula mass of a typical soap is 306.
Now, we’ve calculated the formula
masses of all the substances given in the problem. Comparing the formula masses that
we calculated, we can see that a typical soap is the substance that has the largest
formula mass. So, answer choice (E) is the
correct answer.
