Everyone talks about the concentration of solutions. They may also talk about the concentration of coffee or tea. Everyone has a particular view of what is meant by the concentration of a solution. You must have noticed that whenever you make coffee if you add a lot of powder, you will end up with a concentrated drink, whereas if you add little, it will result in a dilute solution. Therefore, it is essential that you understand what is the concentration of the solution. In this chapter, we will learn about what is meant by the concentration of a solution; we will also see how to find the concentration of a solution and the different methods of expressing concentration of solution.

In an aqueous solution, two parts exist, namely solute and solvent. They are the two basic solution concentration terms that you need to know. We always need to keep an account of the amount of solute in the solution. The amount of solute in the solvent is what is called the concentration of a solution. In chemistry, we define concentration of solution as the amount of solute in a solvent. When a solution has more solute in it, we call it a concentrated solution. Whereas when the solution has more solvent in it, we call it a dilute solution. Now that you understand the concept of what is concentration of solution let's move on to the different methods of expressing concentration.

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The image shows a solution from the most dilute solution to the most concentrated solution.

There are various methods of expressing concentration of a solution. You will usually see Chemists working with the number of moles. Pharmacists will use percentage concentrations instead of the number of moles. Hence it is important to understand all the methods of expressing concentration of solutions.

The concentration of solution formula is given as follows.

Cor S = \[\frac{\text{Weight of solute in grams}}{\text{Volume in litres}}\]

We will also see other methods on how to calculate concentration of a solution based on the different methods of expressing concentrations.

It is expressed in terms of weight in weight. The formula for parts per million is given as follows.

ppm (A) = \[\frac{\text{Mass of A}}{\text{Total mass of solution}}\times 10^{6}\]

It is expressed in terms of mass percentage of solute to the solution. The formula for mass percentage is given as follows.

Mass percentage of A = \[\frac{\text{Mass of component A}}{\text{Total mass of solution}}\times 100\]

e.g. CH3COOH 33% w/w, and H_{2}SO_{4} 98.0% w/w.

It is expressed in terms of volume percentage of solute to the solvent. The formula for volume percentage is given as follows.

Volume percentage of A = \[\frac{\text{Volume of component A}}{\text{Total volume of solution}}\times 100\]

Percentage weight in volume expresses the number of grams of solute in 100 ml of product.

e.g. BaCl2 solution 10% w/v, and H2O2 solution 5-7% w/v.

It is the number of moles of solute contained in 1000 ml of solution. It is a commonly used method for expressing concentrations.

The molality is expressed as the number of moles of a solute contained in 1000 gm of a solvent. The formula for molality is given as follows.

Molality (m) = \[\frac{\text{Mass of solute}}{\text{Mass of solvent in Kg}}\]

We can define it as the number of equivalents of the solute present in the solution, and it is also called as equivalent concentration. The formula for normality is given as follows.

Normality (N) = \[\frac{\text{Weight of solute in grams}}{\text{Equivalent mass\times Volume in litre}}\]

Now that you know how to find the concentration of a solution using various concentration of solution formulas, we will try to solve some concentration of solution questions.

Question 1) 2ml of water is added to 4g of a powdered drug. The final volume is 3ml. Find the mass by volume percentage of the solution?

Answer 1) Given, Mass of solute = 4g

Volume of solution = 3ml

Mass by volume percentage = \[\frac{\text{Mass of solute}}{\text{Volume of solution}}\times 100%\]

= \[\frac{4g}{3ml}\times 100%\]

= 133%

Therefore, the mass by volume percentage is 133%.

Question 2) Many people use a solution of Na3PO4 to clean walls before putting up wallpaper. The recommended concentration is 1.7%(m/v). Find the mass of Na3PO4 needed to make 2.0L of the solution?

Answer 2) Given, Mass/Volume percentage = 1.7%

Volume of Solution = 2000ml

Mass by volume percentage = \[\frac{\text{Mass of solute}}{\text{Volume of solution}}\times 100%\]

1.7 % = \[\frac{\text{Mass of solute}}{2000 ml}\times 100%\]

Mass of solute = 34 g

Therefore, the mass required is 34g.

FAQ (Frequently Asked Questions)

Question 1) Does Solution Concentration Change when Solution Volume Changes?

Answer 1) The answer to this question depends on how we define concentration. If we talk about molarity, then yes it does change. If we take concentration by mass into consideration, it will still change, unless the substance is with an undefined density. That's because the mass of a substance will change with its volume, and so the concentration changes. But, if both the solute and solvent are either increasing or decreasing in volume/mass/moles in an equal ratio, the concentration and molarity will remain the same.

Question 2) How do I Convert from Molarity to a Weight Percentage?

Answer 2) First step is to multiply the molarity by the molar mass of the solute to get grams of solute per litre. The second step is to divide the concentration expressed as grams of solute per litre by the density of the solution in grams per litre. Finally, multiply it by 100% to convert it to percentage.