C4 Stoichiometry
C4.1 Stoichiometry
• 1 Use the symbols of the elements and write the formulae of
simple compounds
• 2 Determine the formula of an ionic compound from the
charges on the ions present
• 3 Deduce the formula of a simple compound from the
relative numbers of atoms present
• 4 Deduce the formula of a simple compound from a model
or a diagrammatic representation
• Stoichiometry is the relationship between the relative quantities of
substances taking part in a reaction or forming a compound
• the study of the measureable relationships that exist in chemical
formulas and chemical reactions.
CHEMICAL FORMULA
• A formula is a combination of symbols and numbers that represents
compounds.
3 different elements, H, P and O.
Contains 3-H, 1-P, 4-O atoms.
1 molecule of H3PO4 contains a total
of 8 atoms.
Subscript 1 is not written in the formulas of compounds.
CHEMICAL FORMULA
• In order to write formula of compounds names of ions
should be known.
• Cations: positively charged metal ions, named same as metal
elements
• Ex :+1 Na+ (sodium), K + (potassium),
H + (hydrogen), NH4 + (ammonium),
• +2 Mg +2 (magnesium), Ca +2 (calcium),
• +3 Al +3 (aluminium)
CHEMICAL FORMULA
• Anions : negatively charged nonmetal ions
• If anion is made of one element : ends with –ide
• Ex: Cl- chloride , F- fluoride, Br- bromide
O-2 oxide, S-2 sulfide
• Compound ions:
• Ex: OH- hydroxide NO3- nitrate
•
SO4-2 sulfate
CO3-2 carbonate
Writing Formulas of Ionic Compounds
• Net charge of ions of a compound must be zero. Total positive charge
should be equal to total negative charge.
• If values of charges are same just write the symbols of ions next to each
other, cation first then anion
• Ex: Na+ and Cl-  NaCl sodium chloride
Mg+2 and O-2  MgO magnesium oxide
• If values of charges are different cross that values:
• 3. Ca+2 and N-3  Ca3N2
• 4. Na+ and CO3-2  Na2CO3
• 5. Ca+2 and NO3-1  Ca(NO3)2
Writing Formulas of Ionic Compounds
• Net charge of ions of a compound must be zero. Total positive charge
should be equal to total negative charge.
• If values of charges are different cross that values:
• Write the formula of compounds between ions given below.
• 1. K+ and Br2. Ca+2 and O-2
3. Ca+2 and N-3
• 4. Na+ and CO3-2 5. Ca+2 and NO3-1
6. NH4+ and CO3-2
•1. and S
+2
•2. Ca and Cl
+3
-2
•3. Al and O
+
-2
•4. Na and CO3
+2
•5. Ca and NO3
+
-2
•6. NH4 and SO4
+
K
-2