29. Moles, Concentration & Volume Calculations

Moles, Concentration & Volume Calculations

Based on the video, here are the notes on calculating the concentration of a solution and relating it to the number of moles and volume.

1. The Concentration Formula

The relationship between moles, concentration, and volume is given by:

• Units: * Volume must be in decimeters cubed (dm3).

  • Concentration is measured in moles per decimeter cubed (mol/dm3).

• Conversion: Since many lab volumes are given in centimeters cubed (cm3), you must divide by 1000 to convert to dm3 (e.g., 800 cm3 = 0.8 dm3).

2. Simple Calculations

• Finding Moles: If you have 0.8 dm3 of a 0.2 mol/dm3 solution:

  • Moles = 0.2 × 0.8 = 0.16 moles.

• Finding Concentration: If you have 0.6 moles in 1.5 dm3:

  • Concentration = Moles / Volume

  • 0.6 / 1.5 = 0.4 mol/dm3.

3. Titration-Style Calculations (The 3-Step Method)

When reacting two solutions (like an acid and an alkali), you can find an unknown concentration using these steps:

Step 1: Calculate moles of the "known" substance

• Convert the volume to dm3 and multiply by the given concentration.

• Example: 0.03 dm3 of 0.5 mol/dm3 KOH gives 0.015 moles.

Step 2: Use the molar ratio from the balanced equation

• Look at the coefficients in the equation to find how many moles of the "unknown" react with the "known".

• Example: If the ratio of KOH to H2SO4 is 2:1, divide the moles of KOH by 2.

  • 0.015 / 2 = 0.0075 moles of H2SO4.

Step 3: Calculate the unknown concentration

• Use the moles from Step 2 and the volume given in the question (converted to dm3).

• Concentration = Moles / Volume

• Example: If the H2SO4 volume was 25 cm3 (0.025 dm3):

  • 0.0075 / 0.025 = 0.3 mol/dm3.

4. Summary Table of Formula Rearrangements