Liquid reagents, whether pure liquids or solutions, are normally measured by volume. You can convert a pure liquid’s volume to number of moles if you know how to convert volume to mass.
- Look up your liquid’s density (expressed in units of “g/mL” or “g/cm3“). Call this “D”.
- Measure the volume of your liquid (in mL) using a graduated cylinder. Call this “V”.
- The weight of your liquid (in g) is given by the product of its density and volume.
weight = DV
Sometimes you have a different problem: you know the mass (in g) of pure liquid that is needed, and you want to calculate the corresponding volume (in mL). In this case, repeat step 1 and calculate the required volume using:
V = weight / D
Solution reagents. The instructions given above do not apply to solutions like “2 M aq. NaOH” or “conc. HCl”. In these cases, you begin with the amount of solute contained in a certain volume of solution, and proceed accordingly. For example, 2 M aq. NaOH contains 2 moles of NaOH in each liter of solution (or, equivalently, 2 mmol NaOH in each mL of solution).