Molecular & Formula Mass - Class 9 Science

Ek Molecule Ka Vazan 🧮

Humne atomic mass dekhi (ek atom ka relative mass). Ab agar ek molecule mein kai atoms hain, toh uska total mass kya hoga? Bahut simple - sabhi atoms ki masses jod do! Isi ko Molecular Mass kehte hain.

📌 Molecular Mass = molecule ke saare atoms ki atomic masses ka jod (sum). Ise bhi u (atomic mass unit) mein likhte hain.

Nikaalne ke 3 steps:

Compound (yaugik) ka formula likho.
Har element (tatva) ke atoms gino (subscript dekho).
Har element ki atomic mass × uske atoms ki sankhya, phir sab jod do.

📋 Yaad rakho (atomic masses): H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5, Ca = 40

🧮 Solved Examples - Molecular Mass

✔ Example 1 - Paani (H₂O):

Atoms: 2 Hydrogen + 1 Oxygen
= (2 × 1) + (1 × 16)
= 2 + 16 = 18 u

✔ Example 2 - Carbon dioxide (CO₂):

Atoms: 1 Carbon + 2 Oxygen
= (1 × 12) + (2 × 16) = 12 + 32 = 44 u

✔ Example 3 - Ammonia (NH₃):

= (1 × 14) + (3 × 1) = 14 + 3 = 17 u

✔ Example 4 - Methane (CH₄):

= (1 × 12) + (4 × 1) = 12 + 4 = 16 u

✔ Example 5 - Sulphuric acid (H₂SO₄):

= (2 × 1) + (1 × 32) + (4 × 16)
= 2 + 32 + 64 = 98 u

✔ Example 6 - Calcium hydroxide, Ca(OH)₂ [bracket wala!]:

Bracket ka matlab: OH ka poora group 2 baar hai. Toh 2 Oxygen aur 2 Hydrogen.
= 40 + 2 × (16 + 1) = 40 + 2 × 17 = 40 + 34 = 74 u

✔ Example 7 - Glucose (C₆H₁₂O₆):

= (6 × 12) + (12 × 1) + (6 × 16)
= 72 + 12 + 96 = 180 u

🧂 Formula Unit Mass (Ionic Compounds Ke Liye)

Yaad hai? Ionic compounds (jaise NaCl) mein alag molecules nahi hote - unme ions ka jaal (lattice) hota hai. Isliye unke liye hum "molecular mass" nahi kehte, balki Formula Unit Mass kehte hain. Nikaalne ka tareeka bilkul same hai!

✔ Example 8 - Sodium chloride (NaCl):

= 23 + 35.5 = 58.5 u

✔ Example 9 - Calcium carbonate (CaCO₃):

= 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100 u

✔ Example 10 - Sodium carbonate (Na₂CO₃):

= (2 × 23) + 12 + (3 × 16) = 46 + 12 + 48 = 106 u

📌 Farak sirf naam ka hai: Molecules wale compounds (H₂O, CO₂) ke liye "molecular mass"; ionic compounds (NaCl, CaCO₃) ke liye "formula unit mass". Calculation dono mein ek jaisa hi hota hai.

⚠️ Common Galtiyan (Inse Bachna!)

❌ Bracket bhool jaana: Ca(OH)₂ mein OH ka poora group 2 baar hai - sirf O ya sirf H ko 2 mat karo. Sahi = 40 + 2(16+1) = 74.

❌ Subscript ignore karna: H₂SO₄ mein O 4 baar hai - 4 × 16 = 64 karna hai, sirf 16 nahi.

❌ Atomic mass galat yaad rakhna: Cl = 35.5 hai (36 nahi), Ca = 40 (20 nahi - 20 toh atomic NUMBER hai!).

✏️ Khud Try Karo (Practice)

Q. Molecular / formula unit mass nikalo:

HCl
MgCl₂
NaOH
H₂O₂ (hydrogen peroxide)

Answers: 1) 1 + 35.5 = 36.5 u 2) 24 + 2(35.5) = 95 u 3) 23 + 16 + 1 = 40 u 4) 2(1) + 2(16) = 34 u

🧠 Chalo Test Karein! (Quick Quiz)

Apne aap ko check karo 👇

Q1. Paani (H₂O) ka molecular mass?

Q2. CO₂ ka molecular mass?

Q3. NaCl ke liye sahi shabd kaunsa hai?

Q4. Ca(OH)₂ ka formula unit mass?

Q5. H₂SO₄ mein oxygen ka contribution kitna hai?

📌 Aage Kya? (Coming Next)

Aakhri lesson mein chemistry ka sabse powerful concept - Mole aur Avogadro number - jisse hum grams ko atoms/molecules ki ginti se jodte hain!