Untitled - Kelly Walsh High School

8 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
Suppose the original question was “How many inches are in 3.5 mi?” We could
use our answer of the number of feet in 3.5 mi and apply the relationship “12 in
1 ft” to do one more conversion:
This step is just like the original conversion of miles to feet. We can now enter
these values into a calculator:
In problems such as the mile to inch conversion, it is simpler to combine the two
calculations into one. In this case, we write the first step:
Now, insert the second step and calculate the answer:
3.5 mi a
18,000 ft a
12 in
18,000 ft a
1ft b 216,000 in 220,000 in (or 2.2 105 in)
3.5 mi a
12 in
b ? in
1ft
5280 ft
b ? in
1mi
5280 ft in
ba12
1mi 1ft b 220,000 in (or 2.2 105 in)
This method allows us to get the answer without calculating an intermediate
value. It also avoids the possibility of too much rounding, which is always a concern
in multistep problems.
Only round your final answer to the proper number of significant figures.
You can work all unit conversion problems by this procedure. In some cases,
such as the miles to feet problem, only one step is necessary. In other cases, such
as the miles to inches problem, more than one step is necessary. However, all
steps are of the same type. You will cancel one unit and get a new unit. This
process continues until the new unit matches the one that you are seeking.
One of the concepts students often see in the introductory material of chemistry
texts is density. Density is defined as the mass per unit volume (density
mass/volume). We treat this relationship as a simple definition and not just a