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ChemistryClass 10CBSE

Why is NaOH's molar mass exactly 40?

Atomic masses: Na=23, O=16, H=1 → 23+16+1 = 40 g/mol exactly. This means 1 mole of NaOH weighs 40 g, making it easy to prepare 1M solutions (40 g per litre).

ChemistryClass 10CBSE

Molecular Mass Calculations

NaOH molecular mass = 40 g/mol:Na(23) + O(16) + H(1) = 40 g/mol FeSO₄·7H₂O (Green Vitriol) = 278 g/mol:Fe(56) + S(32) + 4×O(16) + 7×H₂O(18) = 56 + 32 + 64 + 126 = 278 g/mol Na₂SO₄·10H₂O (Washing Soda / Glauber's Salt) = 322 g/mol:2×Na(23) + S(32) + 4×O(16) + 10×H₂O(18) = 46 + 32 + 64 + 180 = 322 g/mol

ChemistryClass 10CBSE

Synthetic Indicators 

IndicatorTypeIn AcidIn BaseLitmusNaturalRedBlueTurmericNaturalYellowRed-brownPhenolphthaleinSyntheticColourlessPinkMethyl OrangeSyntheticRedYellow

ChemistryClass 10CBSE

Why polar aprotic → SN2

No H-bonding with nucleophile → nucleophile stays "naked" and highly reactive → faster backside attack.Best example: DMSO (dimethyl sulfoxide) most commonly cited polar aprotic solvent in exams.

ChemistryClass 10CBSE

Polar Aprotic Solvents

FeatureProtic SolventsPolar Aprotic SolventsO–H / N–H bondsYesNoH-bond donationYesNoExamplesWater, ethanolDMSO, DMF, acetoneFavoursSN1 reactionsSN2 reactions

ChemistryClass 10CBSE

Conditions for ionic bond formation

Large electronegativity difference (>1.7 Pauling units)Low ionisation energy of the metal (easy electron loss)High electron affinity of the non-metal (eager electron gain)High lattice energy of the product (stabilises the ionic crystal)Both ions achieve noble gas / stable configuration (octet rule)Class 10 example — NaCl:Na (2,8,1) → Na⁺ (2,8) + e⁻ [loses 1 electron] Cl (2,8,7) + e⁻ → Cl⁻ (2,8,8) [gains 1 electron] Na⁺ + Cl⁻ → NaCl (ionic bond via electrostatic attraction)

ChemistryClass 10CBSE

Salt and diabetes

Excess salt is NOT recommended it raises blood pressure, worsening diabetic kidney and heart complications. WHO recommends <5 g/day; diabetic patients should follow a low-sodium diet.

ChemistryClass 10CBSE

Na-22 vs Na-23

Na-23 is the naturally occurring stable form; Na-22 is a radioactive isotope used in PET scans. For all school chemistry: atomic mass of Na = 23 u.

ChemistryClass 10CBSE

Why sodium's mass is 23

Na has 11 protons + 12 neutrons = 23 u. It has only one stable isotope (Na-23) unlike most elements, no weighted average is needed.

ChemistryClass 10CBSE

Na₂CO₃ Is Also Basic

salt of strong base NaOH + weak acid H₂CO₃; CO₃²⁻ hydrolyses to give OH⁻; pH ≈ 11–12.

ChemistryClass 10CBSE

Is NaHCO₃ basic?

Yes justified:Salt of strong base (NaOH) + weak acid (H₂CO₃)HCO₃⁻ hydrolyses in water → produces OH⁻ ionspH of solution ≈ 8.3–8.5 (mildly alkaline)Turns red litmus blue; reacts with HCl to release CO₂Conversion of NaHCO₃ → Washing Soda:Step 1: 2NaHCO₃ --Heat→ Na₂CO₃ + H₂O + CO₂ (heating) Step 2: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O (recrystallisation)

ChemistryClass 10CBSE

Walking on Sand or Ice

On Ice too little friction: The smooth surface cannot provide the backward reaction force (Newton's 3rd Law) needed to push you forward feet slip.On Sand surface gives way: Loose particles shift under foot pressure, absorbing energy without returning a solid reaction force. Each step requires 2–3× more energy than firm ground.Feet hurt on sand because: Calf, arch, and Achilles tendon muscles work 2–3× harder; feet sink and grip sand; constant balance correction strains ankles and knees.Class 9 Core Concept: Walking requires firm ground reaction (Newton's 3rd Law) + adequate friction both are impaired on sand and ice for opposite reasons (too loose vs. too slippery).

ChemistryClass 10CBSE

What are the 7 fundamental units?

The 7 SI Fundamental (Base) Units:UnitSymbolQuantityMetremLengthKilogramkgMassSecondsTimeAmpereAElectric currentKelvinKTemperatureMolemolAmount of substanceCandelacdLuminous intensityNewton, Joule, and Watt are derived units, not fundamental. Kilogram, metre, and second are the fundamental units most directly used to describe a physical body.

ChemistryClass 10CBSE

What pH is eczema skin?

pH of eczema skin = 5.8 to 7.0 Higher than healthy skin's ideal 4.5–5.5. The disrupted acid mantle weakens the skin barrier.

ChemistryClass 10CBSE

What is pH in water?

pH of pure water = 7.0 at 25°C (equal H⁺ and OH⁻ concentrations of 10⁻⁷ mol/L).

ChemistryClass 10CBSE

What is meant by pH value?

pH scale:pH RangeNatureExample0–below 7AcidicLemon juice (~2), Vinegar (~3)Exactly 7NeutralPure waterAbove 7–14Basic/AlkalineBaking soda (~8), NaOH (~14)

ChemistryClass 10CBSE

What is the full name of pH?

Full name of pH of "Potential of Hydrogen" (from German "Potenz") introduced by S.P.L. Sørensen in 1909.Formula: pH = −log₁₀[H⁺]

ChemistryClass 10CBSE

Is there a 7d orbital?

Yes, theoretically but no currently known element uses it in its ground state.

ChemistryClass 10CBSE

2-8-8-18 rule

Maximum electrons per shell = 2n² (Shell 1 = 2, Shell 2 = 8, Shell 3 = 18, Shell 4 = 32)

ChemistryClass 10CBSE

Three rules governing electron configuration

Aufbau Principle fill lowest energy orbitals firstPauli Exclusion Principle: max 2 electrons per orbital, opposite spinsHund's Rule fill degenerate orbitals singly before pairing

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