• Introduction
• Weight/Volume percent solutions
• Volume/Volume percent solutions

Introduction

One of the questions that keep popping up every now among Conservators is, “How do I prepare a percent solution of x solute into a y solvent”? Well, the answer to that is pretty easy, especially in Art Conservation where we mostly tend to use Weight per Volume or Volume per Volume percent concentrations.

Let us first have a look at what each percent concentration means.

Weight/Volume percent solutions

Weight per Volume (w/v) percent solutions are defined as the ratio of the mass of the solute over the total volume of the solution multiplied by 100. The formula is the following:

Where C is the Concentration of the solution, m is the mass of the solute in grams and V is the volume of the total solution. This is usually expressed as grams of solute/100 mL of solution but it could be as easily expressed in L (liters) of solution.

So, what does that tell us? Let me give you an example. Let’s suppose that we need to prepare a 10% w/v solution of NaCl in an aqueous solution. This is of course nothing else than our well-known table salt. What do we do?

1. We weigh 10 grams of NaCl.
2. We fill a beaker with water up to the 50 mL mark and stir our solution.
3. We top up our beaker until the 100 mL mark.

And that’s it! We now have a 10% w/v solution of NaCl in H2O!

What’s confusing for some people though is that they believe that they have to weigh their solute and then add 100 mL of solvent. This is not the case. Weight/Volume percent concentration is the mass of the solute over the total volume of the solution. Always try to keep that in mind.

Here’s another one. Say that you have just found a jar of some solution that does not mention the concentration but it mentions the mass of the solute and the volume of the solution, e.g. 150 g of NaCl in 600 mL of water. How would you calculate the w/v concentration of that one? Well, as it turns out that’s pretty simple! Remember our formula from above? Just plug in the numbers and you have your answer!

Hence, the concentration of your solution is 25% w/v or 25 g/100 mL. And that’s all!

Volume/Volume percent solutions

So, what about Volume/Volume (v/v) percent solutions? How are these prepared? Well, in more or less the same way as the weight/volume percent solutions, apart from the fact that in the case of v/v solutions you measure the Volume of the solute over the total Volume of the solution, such that:

Where C is the Concentration of the solution, V is the Volume of the solute in mL and V is the volume of the total solution. Simply, in this case instead of measuring grams of the solute we are measuring its Volume. For example, let us assume that we have to make a 50% v/v solution of Ethanol (EtOH) in water. We would proceed as follows:

1. We measure 50 mL of Ethanol.
2. We top up our beaker until the 100 mL mark.

And that’s it! More on solutions in the next post where I’ll be talking about dilution. Hint: Have you noticed how our concentration formula above is exactly the same as the density formula?