CHAPTER 3: CHEMICAL FORMULAE AND EQUATIONS A. RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS 1. An atom is extremely _________________. So, it is impossible to weigh an atom in gram. Chemists compared how heavy one atom is to another atom which is taken as standard. The mass of an atom when compared to another is known as __________________________________________. 2. In the beginning, chemists compared the mass of an atom to the mass of a hydrogen atom because it is ____________________. Later on, the ______________ atom was chosen as the standard. Finally, _____________________ is chosen as the standard. 3. Carbon-12 was chosen as the standard because: a) Carbon is the most ______________________ carbon isotope. b) Carbon-12 exists as a _______________ at room temperature and thus can be handled easily unlike hydrogen and oxygen which are in _______________ state. c) Many elements can _______________ with carbon-12. 4. The relative atomic mass of an element is the average mass of an atom of the element when compared with 1 12 of the mass of an atom of carbon-12. Relative atomic mass of an element(RAM) ¿ ❑ 1 12 × t h e mass of an atom of carbon−12 5. Since relative atomic mass is only a comparison so it does not have any _________________. 6. When we need to compare the relative mass of substances made up of molecules, the term relative ________________ mass will be used. Relative molecular mass of a molecule (RMM) 15
Transcript
CHAPTER 3: CHEMICAL FORMULAE AND EQUATIONS
A. RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS
1. An atom is extremely _________________. So, it is impossible to weigh an atom in gram. Chemists compared how heavy one atom is to another atom which is taken as standard. The mass of an atom when compared to another is known as __________________________________________.
2. In the beginning, chemists compared the mass of an atom to the mass of a hydrogen atom because it is ____________________. Later on, the ______________ atom was chosen as the standard. Finally, _____________________ is chosen as the standard.
3. Carbon-12 was chosen as the standard because:a) Carbon is the most ______________________ carbon isotope.b) Carbon-12 exists as a _______________ at room temperature and thus can be handled easily unlike
hydrogen and oxygen which are in _______________ state.c) Many elements can _______________ with carbon-12.
4. The relative atomic mass of an element is the average mass of an atom of the element when compared with
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of the mass of an atom of carbon-12.
Relative atomic mass of an element(RAM)
¿ ❑112
× t h e mass of an atom of carbon−12
5. Since relative atomic mass is only a comparison so it does not have any _________________.
6. When we need to compare the relative mass of substances made up of molecules, the term relative ________________ mass will be used.
Relative molecular mass of amolecule (RMM)
¿ ❑112
× t h e mass of an atom of carbon−12
7. A molecule is made up of a number of atoms. Therefore, the RMM of a substance is calculated by _________________________ the relative atomic masses of all the atoms present in a molecule of the
substance. For example, the RMM of a water molecule with the formula H 2 O is
_________________________.
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8. For ionic substances (substance made up of ions), the term relative _______________ mass (RFM) is used instead.
Exercise: 1. Determine the relative molecular mass (or relative formula mass) of each of the following compounds.(Kindly refer to the Periodic Table of Elements in textbook for the relative atomic mass of the elements.)a) Ammonia, NH3 b) Hydrated barium chloride, BaCl2.2H2O
2. The molecular formula of aspartame is C14H18NxO5. If the relative molecular mass of aspartame is 294, determine the value of x.
3. A compound with the formula UX6 has a relative molecular mass of 352. Calculate the relative atomic mass of element X. (RAM U, 238). Hence, identify element X from the Periodic Table.
B. THE MOLE AND THE NUMBER OF PARTICLES
1. Scientists use the unit ____________ to measure the amount of substances. The symbol of mole is _________.
2. A mole is _______________________________________________________________________________________________________________________________________________________________________.
3. One mole of any substance contains ____________________ particles. The value of 6 .02 ×1023 is called the
_________________________.
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4. Relationship between the number of moles and the number of particles
Number of moles Number of particles
Exercise:1. Calculate the number of particles in
a) 0.2 mol of copper atoms, Cu b) 1.25 mol of sodium ions, Na+
c) 0.03 mol of water molecules, H2O d) 4.5 mol of chlorine gas, Cl2
2. Calculate the number of atoms in a) 1.2 mol of chlorine gas, Cl2 b) 0.25 mol of carbon dioxide gas, CO2
c) 0.75 mol of ammonia gas, NH3 d) 2 mol of sulphur dioxide gas, SO2
3. Calculate the number of ions in the following substances.a) 0.125 mol of sodium oxide, Na2O b) 1.15 mol of magnesium chloride, MgCl2
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c) 0.01 mol of zinc oxide, ZnO d) 2.5 mol of aluminium sulphate, Al2(SO4)3
4. Calculate the number of moles in the following substances.
a) Number of moles of atoms in 9.03 ×1023 aluminium
atoms.
b) Number of moles of ions in 1.96 ×1024 iron(II) ions,
Fe2+.
c) Number of moles of oxygen molecules in
6.02 ×1023 oxygen atoms.d) Number of moles of chlorine atoms in 6.77 ×1023
chlorine gas.
C. THE MOLE AND THE MASS OF SUBSTANCES
1. The mass of one mole of any substance is called its ______________. The unit of molar mass is __________.
2. The molar mass of a substance = the mass of 1 mol of the substance.
3. The molar mass of a substance equals to its respective RAM / RMM / RFM.
4. Relationship between the number of moles and the mass of a substance
Number of moles Mass (g)
Exercise:a) Determine the number of moles of atom in 6g of
carbon.
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b) Determine the number of moles of atom in 4g of copper powder.
c) Determine the number of moles of atom in a balloon with 8g of helium gas.
d) What is the mass of magnesium which contain 0.5 mol of magnesium atom?
e) Determine the mass of 1 mol of lead.
f) What is the mass of aluminium which contain 3.5 mol of Al?
D. THE MOLE AND THE VOLUME OF GAS
1. Most gases are too light to be weighed. So, chemists used to measure their ______________ instead of mass. The ______________________ of a gas is defined as the volume occupies by ____________ mole of the gas.
2. One mole of any gas always has the same _____________________ under the same ____________________ and _________________.
3. The molar volume of any gas is _______________________ at standard temperature and pressure (STP) or _________________________ at room conditions. (1dm3 = _______________cm3)
4. STP refers to temperature ______________ and pressure __________________. Room conditions refer to temperature __________________ and pressure _______________.
5. Relationship between number of moles and volume of gas
Number of moles Volume of gas (dm3)
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Exercise:
1. What is the volume occupied by:a) 44g of carbon dioxide gas at stp?
b) 11g of carbon dioxide gas at room conditions?
c) 64g of oxygen gas at stp?
d) 48g of oxygen gas at room conditions?
2. How many moles of gas related to the volume below? (at stp)a) 67.2 dm3 of SO2 b) 0.880 dm3 of carbon dioxide c) 1000 dm3 of C2H6
Overall Concept of Mole
Number of particles Number of moles Mass (g)
Volume of gas (dm3)
Summary Exercise:
1. Find the number of particles in 24g of magnesium.
2. Find the number of particles on 6g of carbon powder.
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3. Calculate the number of sodium atoms in 46g of sodium metal.
4. Calculate the number of atoms in 80g of calcium.
5. What is the mass of i. 0.1 mol of magnesium? ii. 2.408 × 1023 atoms of
magnesium?iii. 6.02 × 1024 nitrogen
molecules, N2?
6. A sample of calcium oxide (CaO) with the mass of 56g. Calculate:i. No. of moles of CaO ii. No. of moles of Ca2+ iii. No. of moles of O2-
7. A sample of zinc chloride (ZnCl2) with the mass of 34g. Calculate:i. No. of moles of ZnCl2 ii. No. of moles of Zn2+ iii. No. of moles of Cl-
8. A sample of aluminium oxide (Al2O3) with the mass of 5.1g. Calculate:i. No. of moles of Al2O3 ii. No. of moles of Al3+ iii. No. of moles of O2-
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E. CHEMICAL FORMULAE
1. A chemical formula is a representation of a chemical substance using ___________________ for atoms and ___________________________ to show the numbers of each type of atoms that are present in the substance.
2. There are two types of chemical formulae: _______________________ and __________________________.
3. Empirical formula is a chemical formula that shows the __________________________________________ of atoms of each element in a compound.
4. Molecular formula is a chemical formula that shows the ___________________ number of atoms of each element that are present in a molecule of the compound.
Determine the Empirical formula1. Empirical formula of a compound can be determined if:
i. The mass of every element in the compound / molecule is known ORii. The percentage of element in the compound is known.
2. Steps to determine the empirical formula of a compound:i. Find the mass of each element in the compound.
ii. Convert the masses to number of moles of atoms.iii. Find the simplest whole number ratio of moles of the elements.
Exercise:
1. 1.69g of iron is bonded with 0.72g of oxygen, producing iron oxide. Calculate the empirical formula of iron oxide. (Fe, 56; O, 16)
2. 3.12g of element X reacted with 1.44g of oxygen producing X oxide. Determine the empirical formula of X oxide. (O, 16; X=52)
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3. 0.91g of X can produce 1.7g of X oxide after heating it completely. Calculate the empirical formula of X oxide. (X, 27; O, 16)
4. Percentage of element in the calcium carbonate compound is given as,Ca = 40%, C = 12%, O = 48%, calculate the empirical formula of calcium carbonate. (Ca, 40; C, 12; O, 16)
5. One compound with the following composition,Cu = 40%, S = 20%, O = 40%, calculate the empirical formula of the compound. (O, 16; S, 32; Cu, 64).
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6. 2.80g of iron reacted completely with chlorine gas to produce 8.12g of salt. Calculate the empirical formula of the salt formed. (Fe, 56; Cl, 35.5)
7. When 3.2g of M oxide is completely reduced, 2.24g of metal is formed. Calculate the empirical formula of the metal oxide.
8. 42g of element R is bonded with 16g of element T to produce a compound with a formula R 3T4. Calculate the relative atomic mass of element R. (T, 16)
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9. Mass of metal X which reacted with steam = 0.6g, Mass of metal X oxide formed after reacted with steam = 1g. (O, 16; X, 24)a) Calculate the number of moles of metal X which reacted with steam.b) Calculate the number of moles of oxygen reacted.c) Write the empirical formula of metal X oxide.
Determine the Molecular formula1. Molecular formula of a compound can be determined if we know:
i. Its empirical formula.ii. Its relative molecular mass (RMM) or molar mass.
2. Relationship between empirical formula and molecular formula:
Molecular formula =
Exercise:
1. The empirical formula of compound X is CH2 and its RMM is 56. Find the molecular formula of compound X. (H, 1; C, 12)
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2. 2.58g of a hydrocarbon contains 2.16g of carbon. The relative molecular mass of the hydrocarbon is 86. (H, 1; C, 12)a) What is meant by hydrocarbon?
b) Determine the empirical formula of the hydrocarbon.
c) Determine the molecular formula of the hydrocarbon.
3. A hydrocarbon contains 17.23% of hydrogen according to its mass. Relative molecular mass of this hydrocarbon is 58. Determine the empirical formula and the molecular formula of the hydrocarbon. (H, 1; C, 12)
Percentage Composition by Mass of an Element in a Compound
% composition by mass of an element ¿Total RAM of t h e element∈t h ecompound
RMM∨RFM of t h ecompound×100 %
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Example:1. Find the percentage composition by mass of nitrogen in the following compounds:
i. (NH4)2SO4 ii. KNO3
Compare and Contrast Empirical Formula and Molecular Formula1. Similarities:
i. _____________________________________________________________________________________ii. _____________________________________________________________________________________
2. Differences:Empirical formula shows ___________________________________________________________________________________________________________________________________________________________while molecular formula shows ______________________________________________________________________________________________________________________________________________________
Experiment to determine the empirical formula of Magnesium oxide(This method can be used to determine the empirical formulae of reactive metals such as aluminium oxide and zinc oxide.)Material: ___________________________________________________________________________________Apparatus: __________________________________________________________________________________Procedure:1. A ______________________ and its ______________________ are weighed.2. 10 cm of _______________________________ is cleaned with ______________________.3. The _________________________________ is _____________________ loosely and placed in the crucible.
The ____________________ and together with the ________________ and ___________________ are weighed again.
4. The apparatus is set up as shown in the diagram.
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5. The crucible is heated __________________ without its _____________. When the magnesium starts to ________________, the crucible is covered with its _____________.
6. The _____________ of the crucible is lifted from time to time using a pair of tongs.7. When the ______________________ stops ________________, the ____________ is removed and the
_____________ is heated strongly for 2 minutes.8. The __________________ with its lid is allowed to cool to _____________________________.9. The __________________ with its lid and its ________________ are ___________________________.10. The process of _________________, ____________________ and weighing are __________________ until
a constant mass is ________________________.Observation:
Observation Inference1. A __________________________ is seen. ____________________ reacts with ______________.2. ____________________________ released. ___________________ combines with ____________.3. A _________________________ is released. ______________________ is formed.
Results:Description Mass / gMass of _________________ + ______________ XMass of _____________ + __________ + ________________
Y
Mass of __________________ + ______________ + _____________________
Z
Calculation:Element Mg OMass / gNumber of moles of atomsSimplest ratio of moles p q
Empirical formula of magnesium oxide is _____________________.
Discussion / Precaution steps:1. ____________________ is used to remove the ____________________ on the surface of magnesium.
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2. The ________________________ is lifted to allow ____________________from the air reacts with ___________________. The ____________________ is then _____________ quickly to _________________ fumes of _______________________ from escaping.
3. The process of ___________________, _________________ and ___________________ are repeated until a _____________________ is obtained to ensure magnesium react __________________ with oxygen to form __________________________.
4. Chemical equation to represent the reaction between magnesium and oxygen.________________________________________________________________________________________
Experiment to determine the empirical formula of Copper oxide(This method can be used to determine the empirical formulae of less reactive metals such as lead(II) oxide and tin(II) oxide.)
Procedure:1. The ____________________ and the ______________________ are weighed with a chemical balance.2. A little ________________________________ is put into the asbestos crucible (asbestos paper which folded
like the shape of a crucible) and the apparatus is ________________ again.3. The ______________________________ is allowed to pass through the apparatus for a few minutes.4. The gas that comes out from hole X is ________________ into a test tube and tested with a
____________________________. If the _______________ sound is no longer heard, __________________ the hydrogen gas at part X.
5. The copper oxide in the asbestos crucible is _____________________.6. The heating process is _______________ when there is no further change occurring in the asbestos crucible. 7. The _______________________ is flow through the apparatus continuously until the apparatus is
________________ to room temperature.8. When it is cooled, the combustion tube and the asbestos crucible containing the copper oxide is ___________.9. The process of _________________, ________________ and _____________________ is repeated several
times until a constant mass is obtained.
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Observation:Observation Inference1. ____________________ glows The reaction occurs between ____________________
and ____________________.2. The colour of copper oxide changes from
________________ to ______________.The ___________________ metal is formed.
3. ______________________________ are formed at the end of the combustion tube.
__________________ is formed.
Results:Description Mass / gMass of ________________________________ + ____________________________
X
Mass of __________________ + _________________ + _____________________
Y
Mass of _______________________________ + __________________ + ______________________
Z
Calculation:Element Cu OMass / gNumber of moles of atomsSimplest ratio of moles p q
Empirical formula of magnesium oxide is _____________________.
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Discussion / Precaution steps:1. The function of the anhydrous calcium chloride is to _____________________________________________.2. The mixture of zinc and dilute hydrochloric acid in the flask is to ___________________________________.3. The chemical equation for the reaction that occurs in the flask is:
________________________________________________________________________________________4. The combustion tube must be filled up with the hydrogen before the hydrogen is ignited at hole X because
5. The flow of hydrogen gas must be continuous throughout the experiment because i. An ____________________ may occur.
ii. The hot copper produced will react with the _______________ in the air to form ______________________ again.
6. The process of heating, cooling and weighing is repeated until a fixed mass is obtained to ensure all the _________________________ has changed to ____________________.
7. Chemical equation to represent the reaction between copper oxide and hydrogen gas:________________________________________________________________________________________
Ionic formulae1. Ionic formula shows the formulae of ionic compounds [compounds which are made up of positively-charged
ions (cations) and negatively-charged (anions)].
2. When writing the ionic formula, we have to make sure that the total positive charge of the cation must be equal to the total negative charge of the anion.
3. The table below shows the charge of some ions:
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+ 3 Aluminium ion Al3+
Exercise: Write down the chemical formulae of the following ionic compounds.a) Silver nitrate b) Copper (I) chloridec) Sodium thiosulphate d) Ammonium sulphatee) Ammonium phosphate f) Lithium oxideg) Calcium hydroxide h) Lead(II) nitratei) Magnesium carbonate j) Lead (IV) oxidek) Zinc phosphite l) Manganese (II) chloridem) Iron (II) hydroxide n) Sodium nitriteo) Aluminium oxide p) Potassium chromateq) Chromium (III) chloride r) Cobalt(II) chlorides) Copper (II) sulphate t) Potassium bicarbonateu) Nickel (I) chloride v) Iron (III) oxide w) Potassium manganate(VII) x) Lead(II) bromide
Naming of chemical compounds1. The naming of chemical compounds is based on the recommendation of the International Union of Pure and
Applied Chemistry (IUPAC).i. The name of the cation comes first followed by the name of the anion.
ii. Roman numerals are used in the naming of certain metals to distinguish the different types of ions.Example:Copper(I) oxide Copper(II) oxide Iron(III) oxide Lead(I) oxide
iii. The name of the first element in simple compounds is maintained as it is. The name of the second element is added with an ‘ide’.Example:Hydrogen chloride Magnesium bromide Calcium oxide
iv. Greek prefixes are used to show number of atoms of each element in a compound.Example:Carbon monoxide Nitrogen dioxide Sulphur trioxide
Exercise: Write down the name of the following ionic compounds.a) CaCl2 b) Cu(NO3)2
c) CaBr2 d) FeO
e) MgCO3 f) FeSO4
g) MgCl2 h) Mg(OH)2
i) Al2O3 j) (NH4)2SO4
k) CaCO3 l) CuO
m) Na2SO4 n) Zn(NO3)2
o) KNO3 p) Na2O
q) CuSO4 r) NaOH
s) CuNO3 t) BaCO3
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F. CHEMICAL EQUATIONS
1. Chemical equation is a shorthand description of a chemical reaction. It can be written either in words or using chemical formulae.
2. Information obtained from a chemical equation written in chemical symbols:a) Qualitative aspect:
Type of ___________________ Type of ___________________ ________________ state of reactants and products.
b) Quantitative aspect: __________________________________ of reactants and products involved in the chemical
reaction.
3. General symbols used in chemical equations:Symbol Meaning Symbol Meaning+ To separate 2 reactants / products (g)→ Aqueous state
Reversible reaction Gas released(s) ↓
Liquid state Heating / heat energy is given
4. Steps to write a balanced chemical equation:i. Write down the equation in words.
ii. Write down the symbols and formulae for each substance in the reaction. Make sure the symbols and formulae are correct.
iii. Determine the number of atoms for each element.iv. Balance the number of atoms for each element by adjusting the coefficients in front of the chemical
formulae and NOT the subscripts in the formulae.
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Exercise: a) Write a balanced chemical equation for each of the following reactions.
