Chemistry Reference
In-Depth Information
Note: We rounded our answer to 3 significant digits, according to the
rules covered in Lesson 2-4, because the number 7.84 only shows 3 signifi-
cant digits.
It is likely that you will be asked, at times, to use the density formula for
calculations involving a known density. You can solve a problem with only
one unknown, but the unknown can be the mass, volume, or density. Let's
suppose you were asked to solve the following problem.
Example 2
Gold has a density of 19.3 g/cm
3
at standard temperature and
pressure (STP). What would be the mass of a 6.00 cm
3
sample of
this precious metal?
The first thing that you want to do is to list what you have been given,
and what you are asked to find. This will help keep you from making the
mistake of solving for the incorrect unknown. Then, take the original for-
mula for density and isolate the unknown, which in this case is the mass.
You do this by multiplying both sides by “volume.”
Given:
volume = 6.00 cm
3
density = 19.3 g/cm
3
Find:
mass
Density =
mass
volume
volume × density =
Formula:
mass
volume
× volume
After we cancel both volumes on the right-hand side, we get our new
working formula, which is: mass = volume × density. Then we plug in the
numbers, cross out units, solve the calculation, and round.
mass = volume × density
= 6.00 cm
3
× 19.3 g/cm
3
= 116 g
Once again, we rounded to three significant digits, because that is the
least number of significant digits shown in the original problem.
You might also be asked to solve a density calculation for a substance
when the volume is the unknown quantity. Some students have trouble
