Using concentrations of solutions in mol/dm3 (chemistry only) (HT only) revision notes

Understanding Concentration in mol/dm³

Concentration is a key concept in chemistry that describes how much solute is present in a given volume of solution. It is commonly expressed in moles per cubic decimeter (mol/dm³). This topic will explore how to calculate concentration, mass, and volume relationships, and how to apply these concepts in titration experiments.

What is Concentration?

• Concentration is defined as the amount of solute (in moles) divided by the volume of solution (in dm³).
• The formula for concentration is:

Concentration (c) = Amount of solute (n) / Volume of solution (V)

Calculating Amount in Moles

• To find the amount of solute in moles from concentration and volume, use the formula:

n = c × V

• Where:
• n = amount of solute in moles (mol)
• c = concentration in mol/dm³
• V = volume in dm³

Example Calculation

• If a solution has a concentration of 2 mol/dm³ and a volume of 0.5 dm³, the amount of solute is:
• n = 2 mol/dm³ × 0.5 dm³ = 1 mol

Calculating Mass of Solute

• To calculate the mass of solute from concentration and volume, first find the amount in moles, then convert to mass using the molar mass (Mr):

Mass (m) = n × Mr

• Where:
• m = mass of solute in grams (g)
• n = amount of solute in moles (mol)
• Mr = relative formula mass of the solute (g/mol)

Example Calculation

• For a solution with a concentration of 1 mol/dm³ and a volume of 2 dm³, and if the solute is sodium chloride (NaCl) with Mr = 58.5 g/mol:
• n = 1 mol/dm³ × 2 dm³ = 2 mol
• m = 2 mol × 58.5 g/mol = 117 g

Calculating Concentration from Mass

• To find concentration from mass and volume, rearrange the concentration formula:

c = m / V

• Where:
• m = mass of solute in grams (g)
• V = volume of solution in dm³

Example Calculation

• If you have 50 g of NaCl dissolved in 0.5 dm³ of solution:
• c = 50 g / 0.5 dm³ = 100 g/dm³
• To convert to mol/dm³, divide by Mr: 100 g/dm³ / 58.5 g/mol = 1.71 mol/dm³

Titration Calculations

• Titration is a technique used to determine the concentration of a solution by reacting it with a solution of known concentration.
• The relationship between the concentrations and volumes of the reactants can be expressed as:

c₁V₁ = c₂V₂

• Where:
• c₁ and V₁ are the concentration and volume of the known solution
• c₂ and V₂ are the concentration and volume of the unknown solution

Example Titration Calculation

• If 25 cm³ of hydrochloric acid (HCl) is titrated with 50 cm³ of sodium hydroxide (NaOH) solution, and the concentration of NaOH is 0.1 mol/dm³, find the concentration of HCl:
• Convert volumes to dm³: 25 cm³ = 0.025 dm³, 50 cm³ = 0.050 dm³
• Using the formula: c₁ × 0.025 = 0.1 × 0.050
• c₁ = (0.1 × 0.050) / 0.025 = 0.2 mol/dm³

Key Relationships

• Concentration is directly proportional to the amount of solute and inversely proportional to the volume of solution.
• Understanding how to manipulate these relationships is crucial for solving problems in quantitative chemistry.

Practical Applications

• Concentration calculations are essential in laboratory settings, particularly in titrations, where precise measurements are necessary for accurate results.
• Knowledge of concentration is also important in real-world applications, such as pharmaceuticals, where the correct dosage of a drug must be calculated based on concentration.

Conclusion

• Mastering the calculations involving concentrations in mol/dm³ is vital for success in chemistry. Practice with various problems will enhance your understanding and ability to apply these concepts effectively.

Unit 4.3 quantitative chemistry focus

Use Using Concentrations of Solutions in mol/dm3 to connect formula selection, substitution, numerical calculation, final answer, and units. Keep each explanation tied to the exact AQA GCSE Chemistry 8462 subtopic instead of using a broad statement that could fit any calculation page.

Calculation method

For calculation questions, start by naming the formula. Substitute the values with units, carry out the calculation clearly, then give the final answer with the expected unit. For ratio, empirical formula, and molecular formula questions, show how the relationship or simplest whole-number ratio was obtained.

Common boundaries to keep clear

Do not confuse relative atomic mass with relative formula mass, molecules with moles, mass with amount of substance, concentration in g/dm3 with concentration in mol/dm3, percentage yield with atom economy, or coefficients with subscripts.

Practice method

After reading each section, cover the worked example and attempt the formula, substitution, calculation, answer, and unit from memory. Then compare your working with the model method and correct any unit conversion or significant-figure mistakes.