Stoichiometry 1
STOICHIOMETRY
Stoichiometry is the relationship between the amounts of reactants and products in a chemical reaction. It involves the
calculation of quantities of any substance involved in a chemical reaction, based on the quantities of any other substances.
General steps in tackling stoichiometric problems :
Step 1 Balance the equation for the reaction.
Step 2 Convert the given quantity of the substance into its mole by applying one of the following equations :
(a)
Calculations involving number of particles (or formula units)
number of particles
number of moles = -----------------------------Avogadro's constant
(b)
Calculations involving reacting masses
mass
number of moles = -----------------molar mass
(c)
Calculations involving volumes of gases
volume of gas at a certain condition
number of moles of gas = -------------------------------------------------------molar volume of gas under that condition
(d)
Calculations involving concentrations and volumes of solutions
number of moles of solute = molarity x volume of solution
Step 3 Use the coefficients in the balanced equation to set up the appropriate mole ratio. This reacting ratio may be used as a
factor to convert the number of moles of one reactant/product to the number of moles of any other in the reaction.
Step 4 Convert from moles back to the required quantity (using those equations in step 2)
Class Work
Calculate the volume of hydrogen gas produced at room condition when 1.308 g of zinc powder is allowed to react with :
(a)
excess hydrochloric acid
(b)
10 cm3 of 0.5 M hydrochloric acid.
(Given the r.a.m. of zinc = 65.4 ; molar volume of gas at r.t.p. = 24 dm3 )
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Stoichiometry 2
For problems in which the quantities of more than one reactants are given, you must determine if one of them is present in limited
quantity, which is the one that used up first and therefore limits the quantities of the products formed. Any other reactant, in this
case, is said to be present in excess. The reaction is said to have gone to completion when the limiting quantity has been used up.
Chemical Formulae
The empirical formula of a compound is the formula which shows the simplest whole-number ratio of the atoms present. The
molecular formula, on the other hand, shows the actual number of each kind of atoms in the molecule.
Class Work
Find the mass of each kind of atoms in 10.0 g of glucose, C6H12O6 .
Empirical formula of a compound can be calculated from its composition by mass. This, in turn, has to be determined by
experiment, either (a) by elemental analysis; or (b) from combustion data.
Class Work
(a) Elemental analysis of an hydrocarbon shows that it contains 85.7 % of carbon. Determine its empirical formula.
(b) A known mass of magnesium is heated strongly in a crucible, opening and closing
the lid from time to time until the reaction was complete. The content after cooling
was then reweighed and the following results were obtained : (relative atomic
masses : O, 16.0 ; Mg, 24.3)
Mass of crucible + lid
28.09 g
Mass of crucible + lid + Mg ribbon before heating
28.30 g
Mass of crucible + lid + content
28.44 g
(i) Calculate the empirical formula of the oxide of magnesium from the above data.
(ii) Explain why the crucible lid was opened and closed from time to time during heating.
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Stoichiometry 3
Molecular formula of a compound can be deduced from the empirical formula ONLY when its molecular mass is known.
Class Work
A compound X containing carbon, hydrogen and oxygen burned completely in air to form carbon dioxide and water as the only
products. 3.72 g of X gave 5.28 g of carbon dioxide and 3.24 g of water on complete combustion. Find the empirical formula of
X. If its molecular mass was found to lie between 50 and 80, find also its molecular formula.
Ideal Gas Equation
Generalization from Gas Laws and Avogadro’s Law
Class Work
Q 1 The pressure of a fixed mass of gas at 20°C is 1.2 x
Q 2 If the gaseous volume of nitrogen (N2) is three
105 Nm-2. If the gas is reduced to a quarter of its
times that of argon (Ar) at the same temperature
original volume and its temperature is increased to
and pressure, what is the ratio of nitrogen to argon
80°C, what would be the new pressure ?
atoms ?
A. 1.45 x 105 Nm-2
A. 1 : 1
B. 3.98 x 105 Nm-2
B. 2 : 1
C. 5.78 x 105 Nm-2
C. 3 : 1
D. 9.60 x 105 Nm-2
D. 6 : 1
By combining the three Gas Laws as well as Avogadro's law, a more general Ideal Gas Equation is established :
A gas which obeys this equation exactly is called a 'perfect' or 'ideal' gas. In practice, real gases obey the equation very closely at
low pressure and high temperature.
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Stoichiometry 4
Class Work
(a) Calculate the pressure exerted by 1 mole of a gas stored inside a container of volume 100 cm3 under room temperature,
assuming the gas behaves ideally.
(b) Estimate a value for R, based on the fact that the molar volume of gas under standard temperature and pressure is 22.4 dm3.
Determination of the Relative Molecular Mass of a Volatile Liquid
Nowadays, the most accurate and convenient method of measuring relative molecular masses (Mr) is by mass spectrometry.
Originally, Mr were obtained by methods dependent on the gas laws. The ideal gas equation can be transformed into an equation
involving the mass of the gas :
pV = nRT
⇒
pV =
m RT
Mr
⇒
Mr = m R T
pV
Therefore, the molecular mass of a gas or a volatile liquid could be estimated by measuring the volume occupied by a known
mass of the gas/volatile liquid at a certain temperature and pressure.
Class Work
2.0 g of a saturated hydrocarbon gave 450 cm3 of vapour at 200°C and a pressure of 1.01 x 105 Nm-2. Determine its (i)
molecular mass ; and (ii) molecular formula.
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Stoichiometry 5
The apparatus to the right is used to determine the relative
molecular mass of a volatile liquid (e.g. propanone) by
measuring volumes of known masses of the liquid.
Determination of Molar Mass of a Volatile Liquid
steam
Propanone is introduced into the gas syringe by means of a
hypodermic syringe (not shown), which should be injected
into the syringe air space through the rubber cap.
The amount of propanone introduced could be determined by
weighing the hypodermic syringe before and after the
injection.
rubber
cap
gas syringe
hypodermic
syringe
thermometer
steam jacket
steam and
water
Precautions :
(i) air bubbles must be expelled from the hypodermic syringe; and
(ii) await until a steady temperature before injecting propanone.
Dalton's Law of Partial Pressure
The pressure exerted by a gas is due to the combined effect of many collisions between individual molecules and the wall of
the container. Dalton invented the term 'partial pressure' to refer to the contribution to the total pressure made by the
molecules of one particular sort in a mixture of gases.
Dalton's law of partial pressure states that, in a mixture of gases which do not chemically react, the total pressure is the sum
of the partial pressures of the component gases. The partial pressure of a component gas is equal to the pressure which it
would exert if it alone occupied the total volume.
For a mixture of gas A and B ;
P
P
P
total = A + B
It is convenient to introduce a quantity known as mole fraction from which partial pressures could be calculated :
mole fraction of A =
amount of A
-------------------total amount
nA
XA = -------------n +n
A
B
or
As amount is proportional to partial pressures ;
partial pressure of A
amount of A
--------------------------- = ------------------total pressure
total amount
or
PA
nA
-------- = --------ntotal
Ptotal
PA = Ptotal x XA
Class Work
A globe contains oxygen at a pressure of 0.30 atm. Hydrogen is admitted until the total pressure is 0.80 atm, and then nitrogen is
admitted until the total pressure is 0.90 atm. What is (i) the partial pressure ; and (ii) the mole fraction of each gas ?
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