1.1.5
Har measurement asal mein ek standard se comparison hai. Jab tum "5 kg" bolte ho, matlab hai "ek fixed lump of mass jo humne agree kiya hai, usse 5 guna." Science tabhi kaam karta hai jab sab ek same rulers use karein. SI system wahi shared agreement hai — kuch base units ka chhota sa set jisse baki har unit multiplication aur division se banti hai. Chemistry mein hum constantly juggle karte hain kitna stuff hai (mol), kitna bhari hai (kg), kitna hot hai (K), aur kitna push hai (Pa) — aur "answers" (energy J, volume L) bas inhi ke combinations hain.
Definition Chemistry mein use hone wali SI base quantitiesEk base unit independently define hoti hai (doosri units se nahi banti). Saat hoti hain; chemistry inhi chaar par depend karti hai:
Mass: kilogram (kg)
Amount of substance: mole (mol)
Temperature: kelvin (K)
(Length: metre (m) — volume & pressure banane mein use hoti hai)
YE KYU? Chemistry matter reacting ki science hai. Ek reaction describe karne ke liye tumhe pata hona chahiye kitne particles hain (mol), unki mass (kg), unhe drive karne wali thermal energy (K), aur aksar gas ko confine karne wala pressure (Pa, jo m aur kg se derived hai).
Intuition Kelvin kyun, Celsius nahi?Chemistry mein temperature p V = n R T T T = 0 zero thermal motion — absolute zero. Celsius 0 ko paani ke freezing point par rakhta hai (arbitrary!), isliye ratios galat ho jaate hain. Kelvin ka zero physical hai, isliye T multiplicatively meaningful hai.
Conversion: == T K = T ∘ C + 273.15 ==
Intuition Humne mole kyun invent kiyaAtoms bahut chote aur zyada hote hain ek-ek karke count karne ke liye. Lekin reactions atom-by-atom fixed whole-number ratios mein hoti hain. Toh hume ek aisa counting unit chahiye jo (a) particles ki ek huge, fixed number contain kare, aur (b) us number ko ek weighable mass se link kare. Mole dono kaam karta hai.
Ek mole itni amount of substance hai jismein exactly N A === 6.022 × 1 0 23 == N A
Intuition Derived unit ka core ideaEk derived unit tab banti hai jab base units ko ek physics equation mein daala jaata hai. Inhe yaad nahi karte — formula se padh lete hain.
SI coherent volume unit m 3 Litre (L) ek convenient non-SI submultiple hai:
1 L = 1 dm 3 = ( 1 0 − 1 m ) 3 = 1 0 − 3 m 3 = 1000 cm 3 Kyun? 10 cm side wala ek cube. Chemists ise use karte hain kyunki lab glassware usi scale ka hota hai.
Worked example E1 — mass se moles18 g M = 18 g/mol n = m / M = 18/18 = 1 mol Ye step kyun? Molar mass grams-per-mole hai, toh grams ko isse divide karne par grams cancel ho jaate hain, mol bachta hai.
Particles: N = n N A = 1 × 6.022 × 1 0 23 = 6.022 × 1 0 23
Worked example E2 — pressure in SIEk gas 0.05 m 2 2000 N p p = F / A = 2000/0.05 = 40000 Pa = 4 × 1 0 4 Pa = 0.4 bar Ye step kyun? Seedha definition se; phir bar mein convert karne ke liye 1 0 5
Worked example E3 — sirf units se energy checkConfirm karo ki 2 1 m v 2 m = 2 kg v = 3 m/s kg ⋅ ( m/s ) 2 = kg m 2 s − 2 = J 2 1 ⋅ 2 ⋅ 9 = 9 J Ye step kyun? Pehle unit-analysis batata hai ki answer ho sakta hai energy, uske baad number par trust karo.
Worked example E4 — ideal gas, sab SI meinn = 1 mol T = 273.15 K V = 22.7 × 1 0 − 3 m 3 R = 8.314 J mol − 1 K − 1 p p = V n R T = 22.7 × 1 0 − 3 1 ⋅ 8.314 ⋅ 273.15 ≈ 1.00 × 1 0 5 Pa = 1 bar Ye step kyun? Sirf SI units daalo taaki R
Common mistake "Molar mass kg mein hoti hai kyunki kg SI base unit hai."Kyun sahi lagta hai: kg hai SI base mass unit, toh zaahir hai M Fix: SI-coherent M hoti hai kg/mol mein, lekin chemists M g/mol mein table karte hain kyunki atomic masses nice numbers aati hain (water = 18 g/mol, 0.018 kg/mol nahi). Bas mass aur M same mass unit mein rakho aur n
p V = n R T Kyun sahi lagta hai: hum °C mein jeete hain; ye "hai hi" temperature.
Fix: gas law ko ek absolute scale chahiye taaki T ∝
Common mistake "1 L = 1 m³."Kyun sahi lagta hai: dono volumes hain aur 'litre' bada lagta hai.
Fix: 1 m 3 = 1000 L 10 cm cube hai, ek cubic metre ka chhota sa fraction.
Common mistake "Pa aur N/m² alag cheezein hain."Kyun sahi lagta hai: dono alag names lagte hain.
Fix: ye identical hain — Pa bas N·m⁻² ka shorthand naam hai. Har derived unit ki ek equivalent base-unit spelling hoti hai.
Recall Feynman: ek 12-saal ke bachhe ko samjhaoLEGO imagine karo. Base bricks hain mass (kitna bhaari), amount (kitne pieces — ek "mole" matlab ek "6-hundred-thousand-billion-billion pieces ka giant box"), aur kitna garam hai (kelvin, sabse thande possible cold se count karte hue). Badi cheezein jaise energy (joule) aur push (pascal) nayi bricks nahi hain — tum unhe base bricks ko sahi pattern mein click karke banate ho. Ek litre bas ek milk-carton-sized cube hai. Jab bricks pata hain, tum koi bhi label padh sakte ho dekh ke ki kaunsi bricks ek saath snap ki gayi hain.
Mnemonic Chaar base units yaad karne ka tarika"Kids Make Kold Presents" → K g (mass), M ol (amount), K elvin (temp), P a (pressure, though Pa derived hai — ye odd one out tumhe yaad rakhna hai ki derived hai).
Derived ke liye: "Joules Push Litres" — Joule = force×distance, Pascal = force÷area, Litre = length³.
Amount of substance ki SI base unit kya hai? mole (mol)
Avogadro's number N A 6.022 × 1 0 23
1 joule ko SI base units mein express karo. 1 J = 1 kg m 2 s − 2
1 pascal ko SI base units mein express karo. 1 Pa = 1 kg m − 1 s − 2
Celsius ko kelvin mein convert karo. T K = T ° C + 273.15
p V = n R T T absolute hona chahiye taaki wo kinetic energy ke proportional ho; Celsius zero arbitrary hai
1 m³ mein kitne litres hain? 1000 L (1 L = 1 dm³ = 10⁻³ m³)
Moles, mass aur molar mass ko link karne wala formula. n = m / M
Moles aur particle count ko link karne wala formula. n = N / N A
SI mein force ki unit derive karo. F=ma → kg·m·s⁻² = newton (N)
1 bar kitne Pa ke barabar hai? 1 0 5
1 atm kitne Pa ke barabar hai? 101325 Pa
Ek derived unit "derived" kyun hai? ye base units ko ek defining equation se multiply/divide karke banti hai
absolute zero makes T meaningful
