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How to Make a Solution in Chemistry

Jon has a Ph.D. in Physical Chemistry and is a teacher. He is also a father to two daughters who are energetic and fun.

how-to-make-a-solution-in-chemistry

What is a Solution in Chemistry?

In chemistry, a solution is defined as a homogenous mixture of two or more compounds, where one compound is the solvent and the other compounds are solutes. As a rule of thumb, the compound with the greatest mass is considered to be the solute.

The ratio between the solvent and solute is usually denoted in two ways. As molar concentration (moles/liter) or as a percentage ratio of weight and volume, such as weight/weight, weight/volume or volume/volume.

In the following example, I will use a single solute with water as the solvent, but we will then see that the same principles apply for solutions with multiple solutes.

Creating a Chemical Solution By Dissolving a Salt

This is the most common type of solution in general chemistry, where a soluble salt (any soluble ionic compound) is dissolved in a specific amount of water.

1. Determine the Required Volume and Concentration of the Solution

The first step is always to determine just how strong a solution you need to make and how much of it you need. A good rule is to create a little more than you need for your experiment as there will always be some amount lost.

For this example, I am going to assume that we need, for whatever reason, to use three 150ml portions of a 0.20 M KOH solution. This means that would I need a total of 450ml. Because I want to have a little extra, I am going to make 500ml of the needed solution.

2. Determine How to Find the Required Mass of the Solute

Once you know the concentration and amount of the solution you can start to figure out how much of the compound you need to dissolve in the required volume. To do that we use a combination of two equations.

First, we need the equation which shows the relation of the concentration to the molar amount of the solvent and volume of the solution. This equation is

how-to-make-a-solution-in-chemistry

where the concentration has the unit of moles/liter and the volume is in liters.

From this equation, we then see that the molar amount of the solute must first be found. To do that we use the equation that shows the relation to a compounds mass, molar mass and molar amount which is

how-to-make-a-solution-in-chemistry

where the mass is in grams and the molar mass is in grams/mole.

We then see from this equation that I also need the molar mass of the compound I am going to use.

3. Finding the Molar Mass of a Compound.

The molar mass of a compound is simply the combined molar mass of all the elements that the compound is made of. The simplest way to do this is to find the type and amount of each element in the molecule, find the molar mass of each element and then add it all together.

You can find the molar mass of each element from the periodic table. K has a molar mass of 39.1 grams/mole, O has a molar mass of 16.0 grams/mole and H has a molar mass of 1.01 grams/mole.

The molar mass of KOH is therefore 39.1 + 16.0 + 1.01 = 56.1 grams/mole.

Infographic on finding the molar mass of a compound. Released under CC-BY-SA 4.0

Infographic on finding the molar mass of a compound. Released under CC-BY-SA 4.0

4. Calculate the Needed Mass of the Solute

Using these two equations and the molar mass of any compound you can calculate the mass you should weigh for any solution you want to make.

Back to the example. I need 500 ml of a 0.20 M KOH solution. To find the amount I need to weigh out I use the following calculation.

how-to-make-a-solution-in-chemistry

So, to make a 500ml solution of 0.20 M KOH I need to dissolve 5.6 grams of KOH in exactly 500 ml of distilled water.

When I then weigh the KOH I should try to get as close to 5.6 grams as I can, but that does not mean that I need to weigh exactly 5.6000 grams. A weight of 5.588 grams or 5.615 grams is acceptable since the accuracy of the solution only needs to be up to two digits.

5. Mixing the Solute and Solvent Together

Once you have measured out the correct amount of the compound you intend to dissolve you should place it is a volumetric flask. This is a special flask that has a single line denoting the precise location of the flasks labeled volume.

Place the solute in the flask and then fill it halfway up with distilled water. Put the stopper in and shake the flask until the solute has completely dissolved. Next, fill the flask nearly to the line with distilled water and then carefully, with a pipette, add water until you hit the line. Stopper the flask again, shake it a bit, label the flask correctly and you are done.

It's important to dissolve the solute first before adjusting the volume as in many cases the solute can affect the final volume of the solution, either by increasing or decreasing it. As an example, if you mix 50 ml of water with 50 ml of ethanol you end up with an approximately 98 ml solution.

Having Multiple Solutes

If you have more than one solute in your solution then you need to follow steps 1. - 4. for each solute you are using. The final solution will then be a mixture with several different solutes.

For example, we could have made a solution by dissolving 0.10 moles of KOH and 0.20 moles NaCl together in distilled water. We would then label the flask with the respective concentration of each solvent (0.10M KOH + 0.20M NaCl) or the concentration of each ion (0.10 M K+, 0,10 M OH-, 0.20 M Na+, 0.20 M Cl-).

Creating a Chemical Solution Using Mass / Volume Ratio

It's sometimes useful to work with solutions using mass/volume ratios. The steps to making such solutions are for the most part the same, but the calculations are different.

1. Determine the Required Volume and Concentration of the Solution

As before, the first step is to determine just how strong a solution you need to make and how much of it you need.

For this second example, I am going to assume that we need to make 2.0L of a 10% (w/v) NaCl solution.

(w/v) means that the mass of the solute is the numerical value of ten percent of the volume of the solution, or in this case 100 grams per 1000 ml.

10% (w/w) means that we need a solution where 10% of the mass of the solution is from NaCl and 10% (v/v) means that the volume of the solute is 10% of the total volume of the solvent and 10%

2. Determine How to Find the Required Mass of the Solute

Here we can use simple ratio equations. We just need to be careful to use the correct one.

how-to-make-a-solution-in-chemistry

3. Calculate the Needed Mass of the Solute

For this type of solution, we do not need to know the molar mass of the solute. Using these equations you can calculate the mass you should weigh for any such solution you want to make.

Back to the second example. I need 2.0 L of a 10% (w/v) NaCl solution. To find the amount I need to weigh out I use the following calculation.

how-to-make-a-solution-in-chemistry

So, to make 2.0 L of a 10% (w/v) NaCl solution I would need to weigh out 200 grams of NaCl. Again, the accuracy only needs to be on two digits so a weight of 199.5 gr or 200.9 gr is just fine in this case.

4. Mixing the Solute and Solvent Together.

Once you have measured out the correct amount of the compound you intend to dissolve you should place it in a volumetric flask as before and then follow the same steps as discussed earlier.

© 2020 Jon Sigurdsson

Comments

Liz Westwood from UK on January 12, 2020:

This takes me back to chemistry classes, though I confess it was never my strongest subject.

AL from South Equator, East Pacific on January 12, 2020:

Great article, i was never a fan of Chemistry, i never seemed to get the right concentration and volume to be able to produce the desired color indications. Even after leaving the field of Chemistry behind, i still have nightmares.

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