MI Activity 1.1.4

Mole, Molarity, Concerntration & Dilution

By Hana'A Said

Mole

• A mole is a standard scientific unit for counting particles—like a “chemist’s dozen”.

• It represents 6.022 × 10²³ elementary entities (atoms, molecules, ions, or electrons). This number is called Avogadro’s number.

• One mole of any substance contains the same number of particles, even though the mass differs.

Multiple Choice

What is Avogadro's Number? 

• 6.02 x 10²³
• - 6.02 10²³
• 6.02 x 10²²
• 6,020,000,000,000

Multiple Choice

Avogadro's number of representative particles is equal to one_____.

• kilogram
• gram
• kelvin
• mole

Molar Mass

• Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol).

• Calculated by adding the atomic masses of all atoms in the chemical formula (found in the periodic table).

• Examples:

  • H₂O: (2 × 1.008) + 16.00 = 18.02 g/mol

  • NaCl: 22.99 + 35.45 = 58.44 g/mol

Multiple Choice

Calculate the molar mass of CO₂.

• 14 g/mol

• 28 g/mol

• 36 g/mol

• 44 g/mol

Multiple Choice

Calculate the molar mass for Al(OH)₃

• 78.0 g/mol

• 72 g/mol

• 28 g/mol

• 25 g/mol

Conversion

• Grams ↔ Moles:

  moles= mass (g)​ / molar mass (g/mol)

• Moles ↔ Particles:

  number of particles=moles×6.022×1023

Key Insight:

The mole lets chemists translate between the invisible world of atoms and measurable lab quantities.

Multiple Choice

Find the mass of 3.8 mol of H₂O. Round to the nearest tenth.

• 57.9 g

• 54.8 g

• 68.4 g

• 62.3 g

Dilution of Solution

Dilution is the process of reducing the concentration of a solution by adding more solvent, while keeping the amount of solute constant.

Dilution Formula

M1​V1​=M2​V2​

• M₁ and V₁: molarity and volume of the concentrated (stock) solution.

• M₂ and V₂: molarity and volume after dilution.

Example

• Prepare 250 mL of 0.25 M HCl from 1.0 M HCl stock:

  V1​ = (M2xV2)/​M1 ​​= (0.25×250mL)/(1.0) ​=62.5mL

• Measure 62.5 mL of 1.0 M HCl and add water until total volume is 250 mL.

Multiple Choice

How do we make a dilution?

• Add more solute

• Remove solute

• Remove solvent

• Add more solvent

Multiple Choice

As we dilute a solution.....

• The volume increases and the molarity (M) increases

• The volume increases and the molarity (M) decreases

• The volume decreases and the molarity (M) increases

• The volume decreases and the molarity (M) decreases

Multiple Choice

What would the molarity of a solution be if you took 10 mL of a 13M stock solution and made a 300 mL solution?

• 390M

• 230M

• 0.43M

• 0.26M