Molar Concentration and Solution Calculations

Full Notes:

Molar concentration (also called molarity) tells us how many moles of solute are dissolved in one cubic decimetre (dm³) of solution.

It is written using square brackets: e.g [NaCl] = 1.0 mol dm⁻³

This is a key concept for working with amounts of substance in solution and titration calculations.

Formula: n = C × V

Moles, concentration and volume can be linked by the following equation:

This relationship is found in the IB data booklet and is used to solve solution-based problems:

• n = amount of solute in mol
• C = concentration in mol dm⁻³
• V = volume of solution in dm³ (convert from cm³ if needed)

Rearranged versions:

• C = n ÷ V
• V = n ÷ C

Units of Concentration

• mol dm⁻³ is used for calculations involving chemical reactions.
• g dm⁻³ is often used when mass is given instead of moles.

To convert:

• mol dm⁻³ to g dm⁻³: multiply by molar mass (M)
• g dm⁻³ to mol dm⁻³: divide by molar mass (M)

Example: 0.10 mol dm⁻³ of NaCl = 0.10 × 58.44 = 5.84 g dm⁻³

Preparing a Standard Solution

A standard solution refers to a solution made to a known concentration made up to a given volume.

When making a standard solution, accuracy is critical. Choose glassware carefully:

• Volumetric flask: for accurate final volume (e.g., 250.0 cm³)
• Pipette: for transferring a precise volume of solution
• Burette: for titrations where volume must be measured gradually
• Beaker/Conical flask: for mixing but not for accurate measurements

Tips:

• Use a balance for accurate mass of solute.
• Always rinse glassware with the correct solution to avoid contamination or dilution errors.

Example: Preparing a standard solution of sodium hydrogen carbonate

• Weighing: Accurately weigh ~2.10 g of NaHCO₃ using a weighing boat and 2 dp balance. Transfer to a 250 cm³ beaker. Rinse remaining solid into the beaker with distilled water.
• Dissolving: Add ~100 cm³ distilled water and stir to dissolve.
• Transferring and Diluting: Transfer solution to a 250 cm³ volumetric flask using a funnel. Rinse the beaker and funnel with distilled water and add washings to the flask. Fill up to the mark with distilled water using a dropping pipette for the final drops. Stopper and invert several times.

Serial Dilution and Calibration Curve

Serial Dilution

Serial dilution is a technique used to create a range of known concentrations by progressively diluting a solution. It is essential for experiments like colorimetry.

Calibration Curve

Calibration curves enable us to determine the concentration of a solution by comparing a measurable property to known samples.

We plot a measurable property (e.g., absorbance from a colorimeter) against known concentrations (obtained from serial dilutions of a stock solution).

The line of best fit allows the concentration of an unknown sample to be determined by interpolation.

Summary

• Use n = C × V to solve solution calculations.
• Express concentration using square brackets and correct units.
• Convert between mol dm⁻³ and g dm⁻³ using molar mass.
• Accurate preparation depends on careful glassware technique.
• Calibration curves allow determination of unknown concentrations.

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