01Class 11 Chemistry

Some Basic Concepts of Chemistry formulas

Mole concept and concentration formulas from Some Basic Concepts of Chemistry. Nearly all of physical chemistry relies on being quick with these.

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Mole concept

Moles from mass: $\displaystyle n = \frac{\text{given mass}}{\text{molar mass}}$
Moles from particles: $\displaystyle n = \frac{N}{N_A}, \quad N_A = 6.022\times10^{23}\ \text{mol}^{-1}$
Moles of a gas at STP: $\displaystyle n = \frac{V}{22.4\ \text{L}}$
Older value. At 1 bar and 273 K, it's 22.7 L.
Mass per cent: $\displaystyle \text{mass \%} = \frac{\text{mass of element}}{\text{molar mass of compound}}\times100$

Concentration

Molarity: $\displaystyle M = \frac{\text{moles of solute}}{\text{litres of solution}}$
Molality: $\displaystyle m = \frac{\text{moles of solute}}{\text{kg of solvent}}$
Doesn't change with temperature.
Mole fraction: $\displaystyle x_A = \frac{n_A}{n_A + n_B}$
Dilution: $\displaystyle M_1V_1 = M_2V_2$

Where students slip up

Confusing molarity and molality. Molarity uses litres of solution, and molality uses kilograms of solvent.
Not balancing the equation before using mole ratios.
Skipping the limiting reagent check when two reactant amounts are given.

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