How to use C1V1=C2V2 How do the units work? I am trying to do a dilution where I know I have 0.749 g/ mL and I want to use this concentration to calculate the grams of aspirin in each 100 mL solution. I am specifically worried about the units. I want grams out. Is it valid to do the following?
One:
$$C1V1=C2V2$$ 
$$(0.749 g)(1 mL)=(C2 g)(100mL)$$
$$C2=0.00749 g$$

or must I do this?
Two:
$$C1V1=C2V2$$ 
$$(0.749 g/mL)(1 mL)=(C2 g/mL)(100mL)$$
$$C2=0.00749 g/mL$$

The difference being the final units. g vs. g/mL
Concentration has units mass over volume; or particles (moles by convention) over volume.

It's fine to use $g/mL$ for concentration in a basic dilution equation.

$C$ is never a mass.

So your second equation has proper units.

If you want mass of aspirin in a given solution with concentration $C$, the mass is given by $m = C\times V$