Solutions and Concentration Units of Solutions

Concept of a Solution

Every sample of matter with consistent properties and a fixed composition is called a phase. For example, water at room temperature and normal pressure exist as a single liquid phase, that is, all the properties of water are consistent throughout this liquid stage.

If a percentage of sugar is added to this sample of water, the sugar dissolves but the sample remains as a single liquid phase. Nevertheless, the properties and structure of this new liquid phase, now the sugar solution, are different from those of distilled water. As this solution of sugar in water is including 2 compounds (binary solution), so it is a mixture, and given that its properties are consistent, for that reason, it is uniform in character.

For a given solution, the quantity of solute dissolved in a unit volume of solution(or a unit quantity of solvent) is called the concentration of the solution. Solutions consisting of relatively lower concentrations of solute are called dilute solution, whereas those consisting of relatively greater concentrations of solutes are called concentrated solutions.

Concentration Unit of Solutions

The systems used to express the amount of solute in solution are called concentration units of solutions. There are numerous types of concentration units of solutions. They are discussed as follows:

Percentage composition

The amounts of solute and solvent can be expressed in Percentage composition by four different methods.

• a. Percentage weight/weight
• b. Percentage weight/volume
• c. Percentage volume/weight
• d. Percentage volume/volume

(a) Percentage weight/weight

It is the weight of a solute dissolved per 100 parts by weight of the solution. 5% w/w sugar solution will include 5 g of sugar dissolved in 100 g of the solution in water. This solution includes 95 g of water.

Example
Calculate the percentage by weight of NaCI if 2.0g of NaCl is dissolved in 20g of water.

 

(b) Percentage Weight/Volume

It is the weight of a solute dissolved per 100 parts by volume of solution. 10 g of glucose in 100 cm³ of the solution is 10% w/v solution of glucose. The amount of the solvent is not precisely known. In such solutions, the overall volume of the solution is under consideration.

 

(c) Percentage Volume/ Weight

 

It is the number of cm³ of a solute dissolved per 100 g of the solution. If we dissolve 10 cm³ of alcohol in water and the total weight of the solution is 100 g, then it is 10% v/w solution of alcohol in water. In such type of solutions, we don’t know the total volume of the solution.

(d) Percentage Volume/ Volume

It is the volume of a solute dissolved per 100 cm³ of the solution.
This unit of concentration is the finest appropriate to the solution of liquids in liquids. A 12 % alcohol beverage is 12 cm³ of alcohol per 100 cm3 of solution. In such solutions, the total volume of the solution may not be always equal to the number of volumes of solute and the solvent.

Molarity (M)

Molarity is the number of moles of solute dissolved per dm³ of the solution.

Preparation:
To prepare one molar solution of glucose in water, we take 180 g of glucose and add sufficient water to make the total volume 1 dm³ (liter) in a measuring flask.
In the case of one molar solution of sucrose, 342 g of sucrose are dissolved in water to make it 1 dm³. Given that the volume of 342 g of sucrose is greater than 180 g of glucose so the volume of water in 1 molar sucrose solution is less than that of 1 molar glucose solution. Anyways, to calculate the volume of the solvent, we need to understand the density of the solute.
Formula:

Calculate the molarity of a solution containing 20.7g of K₂CO₃ dissolved in 500 cm³ of the given solution.

 

Molality (m)

Molality is the number of moles of solute in 1000 g (1 kg) of the solvent.

Preparation:

In order to prepare molal solutions, we do not have to take any flask. 180 g of glucose, when dissolved in 1000 g of water, provides one molal solution of glucose. The overall mass of the solution is 1180 g. We do not know the volume of the solution. In order to know the volume, we require the density of the solution. For one molal sucrose solution, 342 g of sucrose is dissolved in 1000 g of H₂O.
So, one molal solution of different solutes in water have their own masses and volumes.

Formula:

In order to get the molality of any solution, we utilize the following formula.

The molality of a solution is an indirect expression of the ratio of the moles of the solute to the moles of the solvent. The molal aqueous solution of a solute says glucose or NaOH is water down in comparison to its molar solution. The reason is that in a molal solution the quantity of the solvent is relatively higher. The value of concentration given up the units of molality does not alter with temperature but that of molarity does. The factor is that the volume of liquids is affected by the variation in temperature.

Mole Fraction (x)

This unit of concentration may be for any type, of solution i.e. gas in gas, liquid in liquid or solid in liquid, etc. This unit is also relevant to a solution having more than 2 components. The mole fraction of any component in a mix is the ratio of the number of moles of it to the total number of moles of all the components present.
Let there be 3 parts A, B, C making a solution. The number of moles is n_a, n_b, n_c respectively. If the mole fraction of A, B, and C are denoted by x_a, x_b, x_c respectively. Then,

The sum of the mole fractions of all the elements of a solution needs to amount to one. There are no formal units of mole fraction. Anyways, we in some cases multiply mole fraction by 100 to get mole percent.

Parts Per Million (ppm)

It is defined as the number of parts (by weight or volume) of a solute per million parts (by weight or volume) of the solution. This unit is used for very low concentrations of solutions, e.g. to express the impurities of substances in water.

MCQs on Solutions and Concentration Units of Solutions

 

FAQs on Solutions and Concentration Units: