A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Basic concepts and calculations
CSCA Chemistry · Lesson 2 of 21Moles and calculations
Moles et calculs · المول والحسابات
▶ Watch the video lesson (A7LAM students)
Chemists count particles in moles. One mole is 6.02 × 10²³ particles, and its mass in grams equals the molar mass.
Key ideas
- n = m / M (moles = mass ÷ molar mass).
- Molar mass = sum of atomic masses (H = 1, C = 12, N = 14, O = 16, Na = 23, Cl = 35.5).
- Avogadro's number: Nₐ = 6.02 × 10²³ mol⁻¹.
- Concentration c = n / V (mol/L).
Formulas to know
- n = m / M
- N = n · Nₐ
- c = n / V
Worked example
How many moles are in 36 g of water (H₂O)?
- M(H₂O) = 2 × 1 + 16 = 18 g/mol.
- n = m / M = 36 / 18.
Answer: n = 2 mol
Practice questions
1. Molar mass of NaCl?
35.5 g/mol · 58.5 g/mol ✓ · 23 g/mol · 46 g/mol
23 + 35.5 = 58.5 g/mol.
2. Mass of 0.5 mol of CO₂?
22 g ✓ · 44 g · 88 g · 11 g
M = 44 g/mol, so m = 0.5 × 44 = 22 g.
3. 0.2 mol of salt dissolved in 0.5 L of water. Concentration?
0.1 mol/L · 0.4 mol/L ✓ · 2.5 mol/L · 0.7 mol/L
c = 0.2 / 0.5 = 0.4 mol/L.