Dilution of Solutions Revision Notes for Leaving Cert Chemistry

Dilution of Solutions

What is dilution?

Dilution is the process of reducing the concentration of a solution by adding more solvent (usually water). When you dilute a solution, you're essentially making it weaker by spreading the same amount of solute over a larger volume.

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The key principle to remember is that the amount of solute stays exactly the same - only the concentration changes.

Think of it like adding water to squash - you have the same amount of flavouring, but it tastes weaker because it's spread through more liquid.

Common laboratory reagents

In the laboratory, many chemical reagents are purchased as highly concentrated solutions that need to be diluted before use. The table above shows the typical concentrations of common laboratory chemicals:

Sulphuric acid: 18 M
Nitric acid: 16 M
Hydrochloric acid: 12 M
Ammonia: 16 M
Ethanoic acid: 17 M

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These concentrated solutions are then diluted to the required strength for specific experiments and analyses. Working with these concentrated reagents requires careful handling and precise dilution techniques.

The dilution equation

The fundamental relationship in dilution calculations is:

$\text{Vol}_1 \times M_1 = \text{Vol}_2 \times M_2$

Where:

$\text{Vol}_1$ = initial volume
$M_1$ = initial concentration (molarity)
$\text{Vol}_2$ = final volume
$M_2$ = final concentration (molarity)

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This equation works because the number of moles of solute remains constant during dilution. When you add solvent, the volume increases and concentration decreases, but the actual amount of dissolved substance doesn't change.

The diagram above illustrates this concept perfectly - notice how the same number of particles (representing solute molecules) are present in both flasks, but they're spread over a larger volume in the diluted solution.

Serial dilution

Serial dilution is a step-by-step process where a solution is diluted multiple times in a systematic way. Each step reduces the concentration by the same factor, commonly 1:10 (meaning the solution becomes 10 times more dilute).

This bar chart shows how a 1 M stock solution becomes progressively more dilute through five consecutive 1:10 dilutions:

Stock solution: 1.0 M
Test tube 1: 0.1 M
Test tube 2: 0.01 M
Test tube 3: 0.001 M
Test tube 4: 0.0001 M
Test tube 5: 0.00001 M

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Serial dilutions are particularly useful when you need very dilute solutions or when testing the effectiveness of substances at different concentrations. This technique is commonly used in microbiology, pharmacology, and analytical chemistry.

Dilution factor

The dilution factor tells you how many times more dilute your final solution is compared to the original:

$\text{Dilution factor} = \frac{\text{Total volume}}{\text{Volume of stock solution}}$

For example, if you take 1 cm³ of stock solution and make it up to 10 cm³ total volume, the dilution factor is $\frac{10 \text{ cm}^3}{1 \text{ cm}^3} = 10$ .

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For multiple serial dilutions, you multiply the individual dilution factors together. Three consecutive 1:10 dilutions give an overall dilution factor of $10 \times 10 \times 10 = 1000$ .

Worked examples

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Worked Example 1: Simple dilution calculation

Question: What volume of 12 M HCl solution is required to make 500 cm³ of 3 M HCl solution?

Solution: Using $\text{Vol}_1 \times M_1 = \text{Vol}_2 \times M_2$

$\text{Vol}_1 \times 12 = 500 \times 3$
$\text{Vol}_1 = \frac{1500}{12} = 125 \text{ cm}^3$

Answer: 125 cm³ of concentrated acid would need to be diluted to 500 cm³.

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Worked Example 2: Serial dilution factor

Question: If you perform three consecutive 1:10 dilutions, what is the dilution factor?

Solution: Dilution factor = $10 \times 10 \times 10 = 1000$

Answer: The final solution is 1000 times more dilute than the original.

Key principles to remember

Understanding dilution requires grasping several fundamental concepts:

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Critical Principles:

Adding solvent never changes the amount of solute - it only changes the concentration
Serial dilutions are systematic - each step uses the same dilution ratio
Dilution factor indicates the extent of dilution - a higher number means more dilute
The dilution equation $\text{Vol}_1 \times M_1 = \text{Vol}_2 \times M_2$ is your most important tool for calculations

Exam tips

When tackling dilution problems in exams, keep these strategies in mind:

Always check your units are consistent (cm³ with cm³, dm³ with dm³)
Remember that when diluting, the final concentration is always lower than the initial concentration
For serial dilutions, multiply individual dilution factors to get the overall factor
Show all working clearly in calculations

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Key Points to Remember:

Dilution reduces concentration but keeps the amount of solute constant - you're just spreading it through more solvent
The dilution equation $\text{Vol}_1 \times M_1 = \text{Vol}_2 \times M_2$ is fundamental for all dilution calculations
Serial dilution creates a systematic series of decreasing concentrations by repeating the same dilution ratio
Dilution factor = $\frac{\text{Total volume}}{\text{Stock volume}}$ and tells you how much weaker your solution has become
Common laboratory acids and bases are supplied at high concentrations and must be diluted for most experiments

Dilution of Solutions (Leaving Cert Chemistry): Revision Notes