Formula sheet: Some Basic Concepts of Chemistry

Some Basic Concepts of Chemistry12 min readLesson 1 of 6

1.1 How to use this sheet Every numerical in this chapter passes through one of three bridges: mass to moles, volume to moles, or moles to particles. Write the bridge first, then substitute. The units on both sides must match before you reach for a calculator, and the answer must carry a unit and a sensible number of significant figures. 1.2 The mole bridge The mole is the counting unit. One mole contains the Avogadro number of entities, and the mass of one mole in grams is numerically equal to the atomic, molecular or formula mass in unified mass units.

n = \frac{m}{M}

Moles from mass, m in g and M in g mol^-1

N = n N_{A}

Number of particles from moles, N_A = 6.022 \times 10^{23} mol^-1

V_{STP} = n \times 22.7 L (1 bar) V_{STP} = n \times 22.4 L (1 atm)

Volume of a gas at STP; NCERT uses 22.7 L mol^-1 at 273.15 K and 1 bar, JEE papers use 22.4 L mol^-1 at 1 atm

1.3 Composition and formula mass Mass percentage compares the mass contributed by an element with the whole molar mass. Average atomic mass is a weighted mean over the isotopes, with the fractional abundances adding to one.

mass % of E = \frac{mass of E in one mole of compound}{molar mass of compound} \times 100

Mass percent of element E in a compound

A ˉ = A_{1} x_{1} + A_{2} x_{2} + \dots

Average atomic mass from isotopic abundances A_i as fractions

ppm = \frac{mass of solute}{mass of solution} \times 1 0^{6}

Parts per million by mass

1.4 Concentration terms These four ways of expressing concentration are not interchangeable. Molarity changes with temperature because the solution volume changes; molality and mole fraction do not.

M = \frac{n _{solute}}{V _{solution} ( L )}

Molarity, with volume of solution in litres

m = \frac{n _{solute}}{w _{solvent} ( kg )}

Molality, with mass of solvent in kilograms

x_{A} = \frac{n _{A}}{n _{A} + n _{B}}

Mole fraction of A in a mixture of A and B

M_{1} V_{1} = M_{2} V_{2}

Dilution of a solution, moles of solute unchanged

1.5 Measurement

ρ = \frac{m}{V}

Density, mass per unit volume

^{\circ} F = \frac{9}{5} (^{\circ} C) + 32 K =^{\circ} C + 273.15

Temperature scale conversions

1.6 Units worth memorising Quantity SI unit Chemistry unit you will meet Amount of substance mol mmol, kmol Mass kg g, mg Volume m^3 L, mL, cm^3, dm^3 Density kg m^-3 g cm^-3 = g mL^-1 Pressure Pa bar, atm, mm Hg Molarity mol L^-1 (practical) M Number density m^-3 particles per mol

Exam

The limiting reagent is decided in moles, never in grams. Divide each amount given by its coefficient in the balanced equation; the smallest quotient belongs to the limiting reagent, and every product amount is then calculated from it.

Convert mass to moles before comparing reactants.
Balance the equation before writing any ratio.
Keep Avogadro number as 6.022 x 10^23 when the answer needs particles.
Convert mL to L before using the molarity formula.
Report the final answer to the lower number of significant figures in the data.
Count a gas at STP with 22.7 L mol^-1 (NCERT, 1 bar) and use 22.4 L mol^-1 only when a JEE paper expects it.

in hinglish

Yeh chapter ka poora numerical ek hi bridge se chalta hai: mass se moles, moles se particles, ya gas ke liye volume se moles. n = m/M sabse pehla formula hai; iske baad N = n·N_A aur gas ke liye V = n × 22.7 L (NCERT, 1 bar) ya 22.4 L (JEE, 1 atm). Mass percent = element ka mass / molar mass × 100, aur average atomic mass isotopes ki abundance se weighted mean hota hai. Solution ke sawaal mein molarity M = n/V(L), molality = n/kg solvent, mole fraction x_A = n_A/(n_A+n_B) aur dilution ke liye M1V1 = M2V2 yaad rakho. Measurement mein density = m/V, aur temperature ke liye °F = 9/5 °C + 32, K = °C + 273.15. Significant figures answer ki accuracy batate hain, isliye calculation ke beech mein rounding mat karo, sirf final answer mein karo. Limiting reagent hamesha moles se decide hota hai, grams se nahi; isliye balanced equation pehle likho, phir har reactant ke moles ko coefficient se divide karke chhota quotient dhoondo. Unit likhna mat bhoolo, kyunki unit ke bina answer adhoora hai.