GENERAL CHEMISTRY

Chem. 2010

BATAA EL GAFARY

StoichiometryChemical Arithmetic

The Mole: Is the amount of a substance that contains Avogadro’s No. of elementary units.

6.02x1023Avogadro’s Number

NA

If you had a mole of cats . . .They would create a sphere larger than Earth!

If you had a mole of H2O could you swim in it?

NO!Water molecules are so small

that a mole of H2O = 18ml

Gram Formula Mass Gram Molecular Mass Molar Mass

mass in grams of 1 mole of a substance

1 mole of NaCl = 58g = 6.02x1023 particles of NaCl

1 mole of H2O = 18g = 6.02x1023 molecules of H20

of thein

Ex. What mole ratio of carbon to chlorine must be chosen to prepare the substance CCl4 ? = = Ex.2: How many moles of carbon atoms are required to combine with 2.5 mol Cl to form the substance CCl4 ? 1 mol of C react with 4 mol of Cl X mol of C react with 2.5 mol of Cl = X = 0.625 mol of C atoms

a)In case of monoatomic elements: 1 mole = atomic mass(g) = 6.02x1023 atoms Ex.: a) What is the mass in grams of 2 atoms , Ca ?b) How many moles are in 2 atoms of Ca ?c) How many atoms are in 120 g of Ca ? Ca = 40Solution:d) 40 g of Ca = 6.02x1023 atomsX = 2 atomsx = 1.33X10-22 g of Ca

1 mol of Ca = 6.02x1023 atoms y = 2 atoms y = 3.32 x10-24 mol of Ca40 g of Ca = 6.02x1023 atoms 120 g of Ca = ZZ = 1.81 x1024atoms of Ca

b) In case of compounds or diatomic or polyatomic elements: 1 mole = molecular weight (g) = 6.02x1023 molecules Ex. How many atoms of oxygen are there in 75 g of H2SO4 ? H = 1, S = 32, O = 16 SOLUTION: Molecular weight of H2SO4 = 6.02x1023 molecules 98 g of H2SO4 = 6.02x1023 molecules 75 g of H2SO4 = x x = 4.607x1023 molecules of H2SO4

Each 1 molecule of H2SO4 contains 4 atoms of oxygen Each 4.607x1023 molecules of H2SO4 contains y atoms of oxygen y = 1.84 x1024 atoms of oxygen

Molecular Weight ( Molecular Mass or Molar Mass)

If the substace is composed of molecules, the sum of the atomic masses is called molecular weight

Formula Weight ( Formula Mass ):

the sum of the atomic weights represented by the formula of a substance is its formula weight.

Calculate the formula weight of iron sulphate Fe2(SO4)3 . SOLUTION: 2 atomic weight Fe = 2 ( 55.9) = 111.8 g 3 atomic weight S = 3 ( 32.1) = 96.3 g 12 atomic weight O = 12(16) = 192 g Formula weight of Fe2(SO4)3 = 400.1 g

The relationship between mol and mass A) In the case of monoatomic elements: n = Ex.: How many moles of Ca are there in 120 g of Ca ? Ca= 40 Solution : n = = 3 mol

B) In the case of compound or diatomic or polyatomic elements: n =

Percentage Composition: The percentage composition of a compound is readily calculated from the formula of the compound.

Therefore, the percentage of a given element is 100 times o the mass of the element divided by the mass of a mole of the compound

% by weight = x 100

Chemical Formula:

The empirical ( simplest) formula

The molecular formula

Structural formula

The definite composition of a particular compound is represented by its chemical formula. There are three types of chemical formula: