Expressing the Concentrations of Solutions (Leaving Cert Chemistry): Revision Notes
Expressing the Concentrations of Solutions
Understanding how to express the concentration of solutions is fundamental in chemistry. When we dissolve substances to create solutions, we need precise ways to describe exactly how much solute is present in a given amount of solution.
Understanding solutions
Before exploring concentration methods, let's clarify the basic components:
Solute: This is the substance that gets dissolved. It's typically present in a smaller quantity. For example, when making saltwater, the salt (NaCl) is the solute.
Solvent: This is the substance that does the dissolving, usually present in a larger amount. In our saltwater example, water (H₂O) acts as the solvent.
Solution: This is the final homogeneous mixture created when the solute completely dissolves in the solvent. The particles are evenly distributed throughout, creating a uniform mixture.
Solutions are described as homogeneous because you cannot distinguish between the particles of solute and solvent when looking at the mixture. Everything appears uniform throughout.
Why concentration matters
Chemists need to know the exact amount of solute present in a solution for several important reasons:
- Chemical reactions: The concentration affects how fast reactions occur and how much product forms
- Medical applications: Drug concentrations must be precisely controlled for safety and effectiveness
- Industrial processes: Manufacturing requires accurate concentrations for quality control
- Laboratory analysis: Accurate measurements depend on knowing exact concentrations
Incorrect concentrations can lead to failed experiments, dangerous reactions, or ineffective treatments. Precision in concentration measurements is not just important—it's essential for safety and success.
Methods of expressing concentration
There are several different ways to express concentration, each suited to different situations and applications.
Percentage of solute
Percentage concentrations are widely used in household products, medicine, and industry. There are three main types:
Weight per weight (w/w)
This method compares the mass of solute to the total mass of solution.
Formula: % w/w =
Worked Example: 10% w/w NaCl Solution
A solution labelled 10% w/w NaCl contains 10 g of sodium chloride in every 100 g of solution.
If you have 250 cm³ of this solution:
- Contains: 10 g of salt dissolved in 90 g of water
- Total mass: 100 g
- Percentage: w/w
Weight per volume (w/v)
This method relates the mass of solute to the volume of solution.
Worked Example: 10% w/v NaCl Solution
A 10% w/v NaCl solution means there are 10 g of sodium chloride dissolved in enough water to make 100 cm³ of final solution.
This is practical because it's easier to measure volumes than to weigh entire solutions.
Volume per volume (v/v)
This method is used when both solute and solvent are liquids.
Worked Example: 10% v/v Ethanol Solution
A 10% v/v ethanol solution contains:
- 10 cm³ of ethanol
- Mixed with enough water to create 100 cm³ of final solution
This method is commonly used for alcoholic beverages - wine typically contains 10-12% v/v alcohol.
Parts per million (ppm)
Parts per million is used for extremely dilute solutions, particularly in environmental analysis and water testing.
Key relationship: 1 mg per litre = 1 ppm
This means:
- 1 mg of substance per 1000 g of water = 1 ppm
- 1 mg per million mg = 1 ppm
- 1 mg/L = 1 ppm
Worked Example: Chlorine in Water
If a chemist reports that chlorine concentration in water is 2 ppm, this means there are 2 milligrammes of chlorine in every litre of water.
For a 500 mL bottle of water:
- Chlorine present = mg
Moles of solute per litre of solution (molarity)
Molarity is the most important concentration method for chemical calculations and reactions.
Symbol: M or mol/L (though M is preferred in practice)
Definition: Molarity represents the number of moles of solute dissolved in one litre of solution.
Always use the symbol M together with a chemical formula
Correct examples:
- 1 M NaOH = 1 mole of sodium hydroxide per litre of solution
- 0.5 M HCl = 0.5 moles of hydrochloric acid per litre of solution
Never write just "1 M" without specifying the chemical!
Why molarity is so useful
Molarity is particularly valuable because:
- It directly relates to the number of particles in solution
- It makes chemical calculations straightforward
- It's essential for stoichiometric calculations
- It allows easy comparison between different solutions
Practical considerations
Using volumetric flasks
When preparing solutions with accurate concentrations, chemists use special equipment called volumetric flasks. These are designed to contain a very precise volume of solution at a specific temperature.
Key points about volumetric flasks:
- They have a narrow neck with a graduation mark
- The solution level must align exactly with the mark for accuracy
- They're used to prepare standard solutions for analysis
- Temperature affects the volume, so measurements are made at standard conditions
Solution preparation
When making a solution of known molarity, follow these steps:
- Calculate the mass of solute needed
- Dissolve the solute in a small amount of solvent
- Transfer to a volumetric flask
- Add more solvent until the solution reaches the graduation mark
- Mix thoroughly to ensure homogeneity
Key Points to Remember:
- Solute dissolves, solvent holds: The solute is what gets dissolved; the solvent does the dissolving
- Three percentage types: w/w (mass in mass), w/v (mass in volume), v/v (volume in volume)
- ppm for tiny amounts: Parts per million equals mg/L for very dilute solutions
- Molarity is key: M (moles per litre) is the most important concentration unit for chemical calculations
- Always use chemical formulas with M: Write 1 M NaOH, not just 1 M, to avoid confusion