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1. Chemical equations: writing and balancing equations
• Chemical equation – the symbolic representation of
a chemical reaction in terms of chemical formulas.
• Reactants are starting substances in a chemical
reaction (written on the left hand side before the
arrow), and products are substances that result from
a reaction (written on the right hand side of the
arrow).

2. TO DO:
• Balance the following equations.
(a) _CH3OH + _O2 → _CO2 + _H2O
(b) _P4O10 + _H2O → _H3PO4
• Strategy for balancing chemical equations:
– balance first atoms for elements that occur in only one substance
on each side of the equation.
– the coefficients must be in the smallest whole number possible.

3. Mole concept & Calculations
• 1 Mole of a substance (element, compound) refers to 6.022
x1023 particles (atoms, molecules or ions) of that substance.
• The mass of 1 mole of an element or compound refers to its
relative atomic mass, molecular mass or formula unit mass with
the unit grams respectively.
• Define Relative Atomic mass, Relative Molecular Mass, Relative Formula Unit Mass
and give examples.
• Molar Mass (MM) is refers to the mass of 1 mole of a
substance.
• Give examples by relating to Relative Atomic Mass, Relative Molecular Mass &
Relative Formula Unit Mass.

4. Number of moles (n) = mass (m) / Molar
Mass (MM)
Number particles (molecules, ions or atoms)
= number of moles (n) x 6.022 x1023

5. TO DO:
• Calculate the mass in grams of the following.
(a)0.205 mol Fe (b) 0.79 mol F
• Calculate the number of atoms in 25.7 g Al

6. Mass percentages from the formula
Mass percent - % by mass of each element in a compound
relative to the total mass.
Examples:
• Mass percent of C in CH2O is given by:
C / C + 2H + O * 100%
= [12 / 12 + 2(1) + 16] * 100%
= [12 / 30] * 100%
= 40%
•1.836 g sample of coal contains 1.584 g carbon. Calculate the
mass percent of C in the coal.
• A fertilizer is advertised as containing 14.0% nitrogen (by
mass). How much nitrogen is there in 4.15 kg of fertilizer?

7. Elemental analysis, and determining
formula
• A formula of a compound can be obtained
from its percentage composition.
• Compounds of the nature Cx Hy Oz are usually
analysed by combustion in a furnace where a
known mass is burnt, and then the masses of
CO2 and H2O detected & related to the masses
of carbon and hydrogen.

8. The percentage composition of a compound
leads directly to its empirical formula.
Empirical formula – elements are present in the
simplest whole number ratio of their atoms.
Molecular formula – shows the exact number of
atoms of each element in a molecule (is a
multiple of its empirical formula).
Example: Hydrazine (N2H4), Emp. f. is NH2 but
Mol. f. is N2H4

9. TO DO:
• Malonic acid is used in the manufacture of
barbiturates (sleeping pills). The composition of the
acid is 34.6% C, 3.9% H, and 61.5% O. What is the
malonic acid’s empirical formula?
• An oxide of osmium (symbol Os) is a pale yellow
compound. If 2.89 g of the compound contains 2.16
g osmium, what is its empirical formula?

10. Generally, a molecular formula for an ionic
compound is the same as its empirical formula.
But, sometimes an empirical formula is not the
same as molecular formula especially for large
organic molecules.
To obtain a molecular formula, the molecular
mass of the compound must be given (through
experiments).

11. Worked out example: the empirical formula of a
compound is CH2O. What is the molecular
formula if the molar mass is 240 g/mol?
Molar mass accounted by empirical formula =
30 g/mol.
Solution:

12. Chemical equations and stoichiometry
• We have already mentioned that chemical
reactions can be depicted by a balanced
chemical equation.
• Stoichiometry – the relationship between the
quantities of chemical reactants and products (in
line with the principle of the law of conservation
of matter).
• In a balanced equation, the numbers in front of
each formula in the equation are the
stoichiometric coefficients.

13. Example
• Representation of the reaction of nitrogen and
hydrogen in the Haber process for producing
ammonia:
1N2(g) + 3H2 (g) → 2NH3(g)
• 1 mol of nitrogen reacts with 3 moles of hydrogen
to produce 2 moles ammonia (a balanced chemical
equation gives a mole relationship. The Moles can
be converted to mass or volume for gaseous
compounds.

14. Quantities of substances in chemical
reactions
• Stoichiometric coefficients are sometimes
referred to as molar coefficients.
• From the coefficients,moles of product/s
obtained from any given moles of reactant/s
(and vice-versa) can be calculated.
• Therefore, masses or even volumes of products
and/or reactants can be calculated.

15. TO DO:
• --C4H10(g) + --O2 (g) → --CO2(g) + --H2O(g)
• What is the amount (in moles and mass) of carbon
dioxide produced from 0.31 mol of butane
(assuming a complete chemical reaction)? Balance
the equation.
Solution:
• Overnight question: What mass of Cl2 is required to
react completely with 1.45 g of phosphorus? What
mass of PCl3 is produced?