# A rock has a mass of 20 grams and a volume of 30 milliliters. What is the density of this rock?

##### 1 Answer

#### Explanation:

Every time a problem asks you to find the **density** of a substance, your goal will essentially be to determine the mass of **one unit of volume** of said substance.

That is exactly what density tells you -- the mass associated with *one unit of volume* of a given substance.

In your case, the volume of the sample is expressed in *milliliters*, so one unit of volume will be

Now, your rock has a volume of **independent** of the size of the sample! Think physical property here.

That means that the ratio that exists between the mass and volume of *this sample* will be **identical** to the ratio that exists between the mass and volume of **any sample**.

You can thus say that

#1 color(red)(cancel(color(black)("mL"))) * "20 g"/(30 color(red)(cancel(color(black)("mL")))) = "0.67 g"#

So, if **every milliliter** will have a mass of

#"density" = rho = color(green)(|bar(ul(color(white)(a./a)"0.67 g mL"^(-1)color(white)(a/a)|)))#

I'll leave the answer rounded to two **sig figs**.