Concentration of solutions

Understanding the concentration of solutions is important in the study of chemistry because it helps us know how much of a substance, called a solute, is present in a solvent. A solvent is a fluid that dissolves a solute in the form of a solid, liquid, or gas to form a solution. For example, when salt (the solute) is mixed with water (the solvent), it forms a salt water solution.

What is the solution?

A solution is a type of mixture in which two or more substances are evenly distributed. A solution has two main parts:

• Solvent: A substance that dissolves a solute.
• Solute: The substance that is dissolved.

In everyday life, solutions are commonly used interchangeably. For example, lemon water is a solution in which sugar and lemon juice are the solutes, and water is the solvent.

Measuring concentrations

Concentration is a measure of how much solute is dissolved in a given volume of solvent or solution. There are many ways to express concentration. Let's discuss some common ways:

1. Mass percent (weight percent)

Mass percent is a way of expressing concentration, telling us the mass of solute in a given mass of solution. It is calculated using the formula:

Mass percent (%) = (mass of solute / mass of solution) × 100

For example, if you have a solution containing 5 grams of salt in 95 grams of water, the mass of the solution would be 100 grams (5 grams of salt + 95 grams of water). The mass percentage would be:

(5g / 100g) × 100 = 5%

2. Molarity

Molarity is the number of moles of solute per liter of solution. It is a commonly used concentration unit in chemistry and is given by the formula:

Molarity (M) = moles of solute / liters of solution

For example, if 1 mol of salt is dissolved in 1 liter of water, the molarity would be 1 M (one molar).

3. Molality

Molality measures the number of moles of solute per kilogram of solvent. This measurement is useful when the temperature changes, as it does not change with temperature unlike volume-based measurements. It is calculated using:

Molality (m) = moles of solute / kilograms of solvent

For example, if there are 2 moles of sugar in 0.5 kg of water, the molality would be:

2 moles / 0.5 kg = 4 molal

4. Volume percentage

Volume percent is used for solutions containing a liquid solute and solvent and is the volume of the solute in 100 volumes of the solution. It is defined as:

Volume percent (%) = (volume of solute / volume of solution) × 100

For example, if you mix 20 mL of alcohol with 80 mL of water, the volume of the solution will be 100 mL. The volume percent of alcohol is:

(20 mL / 100 mL) × 100 = 20%

5. Parts per million (ppm) and parts per billion (ppb)

PPM and ppb are very dilute concentrations of substances. PPM refers to the mass of solute in solution per million parts, and ppb refers to the mass of solute in solution per billion parts. PPM is calculated as follows:

PPM = (mass of solute / mass of solution) × 10^6

And PPB is calculated as follows:

PPB = (mass of solute / mass of solution) × 10^9

Example problems

Example 1: Calculating molarity

Suppose you dissolve 29 grams of NaCl (table salt) in water to make 0.5 liters of solution. Calculate the molarity.

First, calculate the moles of NaCl. The molecular weight of NaCl is about 58.44 g/mol.

Moles of NaCl = mass (g) / molecular weight (g/mol) = 29 g / 58.44 g/mol = 0.496 moles

Next, use the molarity formula:

Molarity (M) = moles of solute / liters of solution = 0.496 moles / 0.5 L = 0.992 M

Example 2: Determining molality

Determine the molality of a solution containing 10 g of glucose (C ₆ H ₁₂ O ₆ ) dissolved in 500 g of water.

First, calculate the moles of glucose. The molecular weight of glucose is approximately 180.18 g/mol.

Moles of glucose = mass (g) / molecular weight (g/mol) = 10 g / 180.18 g/mol = 0.0555 moles

Use the molality formula:

Molality (m) = moles of solute / kilograms of solvent = 0.0555 moles / 0.5 kg = 0.111 molal

Practical importance of concentration

Understanding solution concentrations is important in many fields. In medicine, accurately dosing medications depends on accurate concentrations. Environmental scientists monitor pollutants in air and water using concentration units such as PPM. In cooking, chefs vary solution concentrations to create desired flavors and textures.

Visualization of solution concentrations

Imagine you are making a sweet drink. You start by adding some sugar to a glass of water. Stir it and it becomes a solution. As you add more sugar, its concentration increases. Visualize it with the diagram below:

The blue segments represent dissolved solutes, which become smaller in proportion as the concentration increases. This visualization helps to understand how concentration affects the composition of the solution.

Conclusion

In short, the concentration of solutions involves various methods for expressing how much solute is present in a given volume of solvent or solution. Understanding how it is calculated and represented is essential for working in many science and industry fields. Practice using these calculations to further understand their applications, and apply these principles in your studies or experiments. Learning how to measure and adjust concentrations is a fundamental skill in chemistry that has wide applications beyond the classroom.

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