Basic concepts of chemistry

quick review

significant figures

• all digits are significant but the leading zeroes

• zeroes to the right side of decimal are significant

• in a notation, N x 10^a, significant numbers are counted only in N.

LAW OF CONSERVATION OF MASS

• mass cannot be created, nor destroyed. only shared.

• the mass of products = mass of reactants

LAW OF DEFINITE PROPORTIONS

a chemical compound always contains its component elements in fixed ratio by mass, regardless of the source.

LAW OF MULTIPLE PROPORTIONS

when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

AVERAGE ATOMIC MASS

If an element exists in two isotopes having atomic masses- m1 and m2, in the ratio x:y, then;
avg atm mass= m1x1 + m2y / (x + y)

MOLECULAR MASS

(a) For atom →1 g →1 g atom = 1 mole.

For molecule →1 g →1 g molecule = 1 mole.

(b) 1 amu or 1 u = 1/12th of the mass of an atom of C- 12 atom =1.66×10−27 Kg.

MOLECULAR WEIGHT

vapour density x 2

VAPOUR DENSITY

density of gas / 2

number of moles of molecules

given mass (g) / molar mass

number of moles of gases

volume of gas (litre) / 22.4 L

number of moles of atoms

given mass / atomic mass

MASS OF X ATOMS

atomic mass / avagradro’s number

relation between empirical and molecular formula

MF = n x EF, where n is integer

n = MF / EF

limiting reagent

gets used completely in the reaction

percentage composition

(mass of element in 1 mol of comp / molar mass of compound) x 100

% impurity

take out % composition, and minus it from 100.

dilution wrt N, M, m

• if a soln is DILUTED use :

M₁ x V₁ = M₂ x V₂

• N = Molarity x N-factor

for questions such as:

how much X element is needed to combust Y completely?

1. write equation and balance it

2. write ratios underneath

3. take out moles of Y

4. multiply moles of Y with ratio of X

5. multiply (4) ka ans with 22.4 if asked volume at STP

N- Factor

• it gives the number of moles of H+ ions exchanged per mole of the substance in acid-base reactions, or the number of moles of electrons transferred in redox reactions

• used to calculate equivalent weight in reactions.

terms like decinomal and seminomal

mean 1/10 and ½

calculating equivalent mass of X in an equation

1. calc eq factor

2. eq mass = mm/eq factor