CHEMISTRY CLASS OF F4A

Captions showing Form 4 A students are having Chemistry class - calculation of empirical formula and molecular formula by teacher, Pn Habshah binti Abdul Kudus.

EMPIRICAL AND MOLECULAR FORMULA

Key Concepts

• Empirical Formula of a compound shows the ratio of elements present in a compound.
• Molecular Formula of a compound shows how many atoms of each element are present in a molecule of the compound.
• The empirical formula mass of a compound refers to the sum of the atomic masses of the elements present in the empirical formula.
• The relative molecular mass (formula mass, formula weight or molecular weight) of a compound is a multiple of the empirical formula mass.
  RMM = n x empirical formula mass
• Empirical Formula can be calculated from the percentage (or percent) composition of a compound.

Molecular formulas and empirical formulas are sometimes the same:

The molecular formula for water is H₂O, a 2 to 1 ratio.
Since the subscripts will not reduce H₂O is also water's empirical formula.

Molecular formulas and empirical formulas are not always the same:

The molecular formula for hydrogen peroxide is H₂O₂, a 1 to 1 ratio.

The formula is correct. Remember that peroxide is a polyatomic ion with a charge of -2.

Reduce the subscripts to the lowest form.
The empirical formula for hydrogen peroxide is HO, a 1 to 1 ratio.

Calculating Empirical Formulas:

Subscripts in a chemical formula are usually thought of as a ratio of atoms.
Subscripts can also be thought of as a ratio of moles . . . . .

a mole being the number of atoms needed
to equal the atomic mass number of an element.

To determine an empirical formula, one must determine the mole ratio.

SCIENTIFC METHOD

Key Info

• The scientific method is a way to ask and answer scientific questions by making observations and doing experiments.
• The steps of the scientific method are to:
  • Ask a Question
  • Do Background Research
  • Construct a Hypothesis
  • Test Your Hypothesis by Doing an Experiment
  • Analyze Your Data and Draw a Conclusion
  • Communicate Your Results
    
    
• It is important for your experiment to be a fair test. A "fair test" occurs when you change only one factor (variable) and keep all other conditions the same.

WHAT IS CHEMISTRY ?

Question: What Is Chemistry?
Answer: If you look 'chemistry' up in Webster's Dictionary, you'll see:

"chem·is·try n., pl. -tries.
1. the science that systematically studies the composition, properties, and activity of organic and inorganic substances and various elementary forms of matter.
2. chemical properties, reactions, phenomena, etc.: the chemistry of carbon.
3. a. sympathetic understanding; rapport. b. sexual attraction.
4. the constituent elements of something; the chemistry of love. [1560-1600; earlier chymistry]."

My glossary definition is the short and sweet, "scientific study of matter, its properties, and interactions with other matter and with energy".

An important point to remember is that chemistry is a science, which means its procedures are systematic and reproducible and its hypotheses are tested using the scientific method. Chemists, scientists who study chemistry, examine the properties and composition of matter and the interactions between substances. Chemistry is closely related to physics and to biology. As is true for other sciences, mathematics is an essential tool for the study of chemistry.

MOLE CONCEPTS

Mole is abbreviated to mol and given the symbol n

1 mole contains the same number of particles as there are in 12g of carbon-12 atoms by definition.
This number is called Avogadro's number or Avogadro's constant (NA) and is equal to 6.022 x 10 23 particles.

1 mole of a pure substance has a mass in grams equal to its Relative Atomic Mass (RAM) or its Relative Molecular Mass (RMM) [also known as molecular weight (MW) or Relative Formula Mass (RFM) or formula weight (FW)].This is often referred to as the molar mass.

1 mole of an ideal gas has a volume of:
22.4 litres (22.4L) at S.T.P.
[Standard Temperature and Pressure, 0 oC (273K) and 101.3kPa (1 atm)]

24.47 litres (24.47L) at S.L.C
[Standard Laboratory Conditions, 25 oC (298K) and 101.3kPa (1atm)]

Avogadro's Number (NA)
1 mole of atoms or molecules contains 6.022 x 10 23 atoms or molecules

THE 6 BRANCHES OF CHEMISTRY

Chemistry is the science of materials, their composition and structure, and the changes they undergo. Quite a broad topic! Chemistry is such a large discipline that it is generally divided into six main branches:

* Analytical chemistry is concerned with the separation, identification, and composition of materials.

* Organic chemistry is the chemistry of most carbon compounds.

* Inorganic chemistry is (you guessed it) the chemistry of materials other than those classed as organic.

* Physical chemistry involves the study of the physical characteristics of materials and the mechanisms of their reactions.

* Biochemistry includes the study of materials and processes that occur in living things.

* Nuclear chemistry involves the study of subatomic particles and nuclear reactions.

FORM 4 A

Today we are doing an experiment to find the empirical formula of magnesium oxide