REVIEWTOPIC ONE: STOICHIOMETRIC RELATIONSHIPS
A. KEY DEFINITIONS
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relative atomic mass(Ar)- the average mass of one atom of the element compared to 1/12 the mass of an atom of
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relative molecular mass(Mr)- the sum of the average masses of the atoms in the molecule compared to 1/12 the mass of
an atom of
12
12
C
C
B. YOU SHOULD KNOW:
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The mole(mol) can be considered as a collection containing a very large number of objects (6.02 x 10 23 to be precise), in
the same way that one dozen is a collection of 12 objects.
The mole can apply to particles, atoms, ions and so on.
Avogadro’s constant (L) is 6.02 x 1023 entities per mol, and there are this number of atoms, ions and so on in one mole.
The definitions of relative atomic mass (Ar) and relative molecular mass(Mr)
An empirical formula is the simplest ratio of the atoms of each elements in a compound.
A molecular formula is the actual number of atoms of each element in a compound.
The symbols of the elements
State symbols: solid (s), liquid (l), gas (g) and aqueous solution (aq), which means dissolved in water
The theoretical yield is the maximum amount of product that can be made and is often different from experimental yield
Avogadro’s law states that one mole of any gas will occupy the same volume if the temperature and pressure are the
same
At standard temperature and pressure, STP(273 K and 100 kPa), the molar volume is 22.7 dm3
the solute is the substance that dissolves
the solvent is a liquid that dissolves the solute
a solution forms when a solute dissolves in a solvent
concentration depends on the amount of solute dissolve and can be measured in gdm -3 and mol dm-3 (1 dm3 = 1000 cm3
= 1 L)
C. YOU SHOULD BE ABLE TO:
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Use the mole concept and Avogadro’s constant to calculate the number of particles or amount of substance
Calculate the amount of substance in moles (mol), molar mass (mass of one mole) in gmol -1 and mass in g if given
sufficient information
Balance chemical equation using whole numbers
Identify mole ratios of reactants and/or products in a chemical equation
Distinguish between coefficients (large numbers in front of formulas) and subscripts(small numbers within formulas)
Identify the limiting reactant in an equation, usually by working out which reactant is in excess(some of this reactant will
be left over)
Solve problems using the ideal gas law equation, PV = nRT
Solve problems using concentration (mol dm-3) = amount(mol)/volume of solution dm3)
D. BE PREPARED
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Use the triangle,
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When reporting relative atomic mass and relative molecular mass be sure to leave out units.
where n = amount in mol, m = mass in g and
M = molar mass in g mol-1
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Set out your working for an empirical formula in a table:
Carbon
Hydrogen
Mass(g)
85.6
14.4
-1
Molar Mass(gmol )
12.01
1.01
Moles
7.13
14.3
Ratio
7.13/7.13 = 1
14.3/7.13 = 2
Formula
C
H2
*The empirical formula is CH2.
When dealing with gases, always use the temperature in kelvins, which is °C + 273.
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Use
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When calculating the concentration of a solution, volume needs to be in dm 3 to match the concentration units of mol
dm-3.
cm3 is converted to dm3 by dividing by 1000.
Use the triangle:
Where:
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n = amount in mol
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C = concentration in mol dm-3
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V = volume in dm3
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when solving problems involving a fixed mass of an ideal gas.
PROBLEM SET #1(TOPIC 1: QUANTITATIVE CHEMISTRY)
1.a)Determine the empirical formula of a hydrocarbon this is comprised of 85.6% carbon and 14.4%
hydrogen.
b) If the molar mass of the compound is 70.2 g mol-1, what is its molecular formula?
2. Write a balanced chemical equation including state symbols for the reaction between aqueous solutions of sulfuric acid and
sodium hydroxide.
3. A mass of 4.38 g of calcium carbonate was added to 100 cm3 of 0.500 mol dm-3 hydrochloric acid.
a) Provide and equation for the reaction that occurs.
b) Determine the limiting reactant.
c) Calculate the volume of carbon dioxide produced, measured at 20°C and 1.02 x 10 5 Pa.
4. In a titration, 25.0 cm3 of phosphoric acid was placed in a conical flask with a suitable indicator. It was found that 29.4 cm 3 of
0.150 mol dm-3 sodium hydroxide was need for complete reaction according to this equation:
3NaOH(aq) + H3PO4(aq) --> Na3PO4(aq) + 3H2O(l)
Calculate the concentration, in g dm-3, of the phosphoric acid.
5. A student reacted some salicylic acid with excess ethanoic anhydride. Impure solid aspirin was obtained by filtering the
reaction mixture. Pure aspirin was obtained by recrystallization. The following table shows the data recorded by the student.
Mass of salicylic acid
3.15 ± 0.02 g
Mass of pure aspirin obtained
2.50 ± 0.02 g
i) Determine the amount, in mol, of salicylic acid, C6H4(OH)COOH, used. (2)
ii) Calculate the theoretical yield, in g, of aspirin, C6H4(OCOCH3)COOH. (2)
iii) Determine the percentage yield of pure aspirin. (1)
6. The value of x in Fe(NH4)2(SO4)2•xH2O can be found by determining the amount, in moles, of sulfate in the compound.
A 0.982g sample was dissolved in water and excess BaCl2(aq) was added. The precipitate of BaSO4 was separated and dried and
found to weigh 1.17g.
a) Calculate the amount, in moles of BaSO4 in the 1.17g precipitate. (2)
b) Calculate the amount, in moles, of sulfate in the 0.982g sample of Fe(NH 4)2(SO4)2•xH2O. (1)
c) Calculate the amount, in moles, of iron in the 0.982g samples of Fe(NH 4)2(SO4)2•xH2O. (1)
d) Determine the mass of the following present in the 0.982g sample of Fe(NH 4)2(SO4)2•xH2O. (3)
(i) Iron
(ii) Ammonium
(iii) Sulfate
e) Use your answer form part d) to determine the amount, in moles, of water present in the 0.982g sample of
Fe(NH4)2(SO4)2•xH2O. (2)
f) Determine the amount, in moles, of Fe(NH4)2(SO4)2 and hence the value of x. (2)