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Counting atoms and molecules
When conducting a chemical reaction, it is often important to mix reactants in the correct proportions. This prevents contamination of the products by wasted reactants.
However, atoms are very
small and impossible to count out. In order to estimate the number of atoms in a sample of an element, it is necessary to find their mass.
The mass of an atom is quantified in terms of
Relative atomic mass
The relative atomic mass (Ar) of an element is the mass of one of its atoms relative to 1/12 the mass of one atom of
carbon-12.
Most elements have more
than one isotope. The Ar of
the element is the average mass of the isotopes, taking into account the abundance of each isotope. This is why the
Ar of an element is frequently
average mass of an atom × 12 mass of one atom of carbon-12 relative atomic mass
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Relative molecular mass
Example: what is the Mr of H2SO4?
(2 × H) + (1 × S) + (4 × O)
1. Count number of atoms
(2 × 1.0) + (1 × 32.1) + (4 × 16.0)
2. Substitute the Ar values
2.0 + 32.1 + 64.0 = 98.1
3. Add the values together
The relative molecular mass (Mr) of a covalent substance is the mass of one molecule relative to 1/12 the mass of
one atom of carbon-12.
Mr can be calculated by adding together the masses of
Relative formula mass
The equivalent of relative molecular mass for an ionic
substance is the relative formula mass.
This is the mass of a formula unit relative to 1/12 the mass
of one atom of carbon-12. It is calculated in the same way as relative molecular mass, and is represented by the same
symbol, Mr.
Example: what is the Mr of CaCl2?
(1 × Ca) + (2 × Cl)
1. Count number of atoms
(1 × 40.1) + (2 × 35.5)
2. Substitute the Ar values
40.1 + 71.0 = 111.1
Avogadro’s law
In 1811 the Italian scientist Amedeo Avogadro developed a theory about the volume of gases.
Avogadro’s law:
Equal volumes of different gases at the same pressure and temperature will contain equal numbers of particles.
For example, if there are 2 moles of O2 in 50cm3 of oxygen
gas, then there will be 2 moles of N2 in 50cm3 of nitrogen
gas and 2 moles of CO2 in 50cm3 of carbon dioxide gas at
the same temperature and pressure.
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Molar volumes of gases
If the temperature and pressure are fixed at convenient standard values, the molar volume of a gas can be
determined.
Standard temperature is 273K and pressure is 100kPa.
At standard temperature and pressure, 1 mole of any gas
occupies a volume of 22.7dm3. This is the molar volume.
Example: what volume does 5 moles of CO2 occupy?
volume occupied = no. moles × molar volume = 5 × 22.7
Ideal gas equation
The ideal gas equation relates pressure, volume, number of moles and temperature for a gas.
pV
=
nRT
p = pressure in Pa
V = volume in m3
How is the number of moles in a gas at other temperatures and pressures calculated?
n = number of moles
R = gas constant: 8.31JK-1mol-1
T = temperature in Kelvin
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Ideal gas equation: converting units
It is very important when using the ideal gas equation that the values are in the correct units.
The units of pressure, volume or temperature often need to be converted before using the formula.
Pressure
to convert kPa to Pa: × 1000
Volume
to convert dm3 to m3:
to convert cm3 to m3:
÷ 1000 (103)
÷ 1000000 (106)
Temperature
Types of formulae
The empirical formula of a compound shows the
relative numbers of atoms of each element present,
using the smallest whole numbers of atoms.
The molecular formula of a compound gives the actual
numbers of atoms of each element in a molecule.
For example, the empirical formula of hydrogen peroxide
is HO – the ratio of hydrogen to oxygen is 1:1.
The molecular formula of hydrogen peroxide is H2O2 – there
Percentage by mass
Elemental analysis is an analytical technique used to
determine the percentage by mass of certain elements
present in a compound.
To work out the empirical formula, the total mass of the compound is
assumed to be 100g, and
each percentage is turned into a mass in grams.
If necessary, the mass of any elements not given by
Calculating molecular formulae
The molecular formula can be found by dividing the Mr by
the relative mass of the empirical formula.
Example: What is the molecular formula of hydrogen peroxide given that its empirical formula is HO and the Mr is 34?
empirical formula mass = H + O = 1.0 + 16.0 = 17
1. Determine relative mass of empirical formula:
2. Divide Mr by mass of empirical formula to get a multiple:
HO × 2 = H O
3. Multiply empirical formula by multiple:
relative molecular mass 34
mass of empirical formula 17
Balancing equations
An important principle in chemical reactions is that matter cannot be created or destroyed. It is important that symbol equations are balanced.
A balanced equation has the same number of each type of atom on each side of the equation.
Na + Cl
2
NaCl
1 sodium 2 chlorine 1 sodium 1 chlorine
Balanced:
This shows that two moles of sodium react with one mole of chlorine to make two moles of sodium chloride.
2Na + Cl
2
2NaCl
2 sodium 2 chlorine 2 sodium 2 chlorine
Balancing ionic equations
Equations containing ions should have the same overall charge on each side in order to be balanced.
This can be achieved by balancing the equation in the normal way:
Ca
2++ Cl
-→ CaCl
22 calcium 1 chloride 2 calcium 2 chloride
+1 charge no charge
2 calcium 2 chloride 2 calcium 2 chloride
Ca
2++ 2Cl
-→ CaCl
2no charge no charge
State symbols
State symbols are letters that are added to a formula to indicate what state each reactant and product is in.
The four state symbols are:
These are added after the formula in brackets and subscript. For example:
Calculating reacting masses
To calculate the mass of a product given the mass of a reactant, use the following steps:
1. Calculate no. moles of reactant:
no. moles = mass / Mr
2. Determine mole ratio of reactant to product:
ensure the equation is balanced
3. Calculate no. moles of product:
use the mole ratio
4. Calculate mass of product:
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Reacting masses example
What mass of sodium chloride is produced if 2.30g of
sodium is burnt in excess chlorine?
3. Calculate
no. moles of NaCl:
0.100 moles Na = 0.100 moles NaCl
4. Calculate
mass of NaCl:
mass = moles × Mr
= 0.100 × 58.5
= 5.85g
1. Calculate
no. moles of Na:
no. moles = mass / Mr
= 2.30 / 23.0 = 0.100
2. Determine mole
ratio of Na to NaCl:
2Na + Cl2 2NaCl ratio = 2:2
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What is concentration?
The concentration of a solution is a measure of how much
solute is dissolved per unit of solvent.
amount of solute is measured in moles
volume of solvent is measured in dm3
concentration is measured in moldm-3.
concentration = amount of solute / volume of solvent
Volumes are often expressed in cm3, so a more useful
equation includes a conversion from cm3 to dm3.
concentration = (no. moles × 1000) / volume
Standard solutions
A standard solution is a solution of known concentration.
Standard solutions are made by
dissolving an accurately weighed mass of solid in a known volume of solvent
using a volumetric flask.
The volumetric flask has a thin neck, which is marked with a line so it can be filled accurately to the correct capacity.
The standard solution can then be used to find the
concentration of a second solution with which it reacts.
A titration is a procedure used to identify the concentration of a solution by reacting it with a solution of known
concentration and measuring the volume required for a complete reaction.
Once the number of moles for the solution is known, the concentration can be easily calculated.
The number of moles in the standard solution is calculated. Using a
balanced equation for the reaction, the number of moles in the solution of unknown concentration can also be calculated.
Titration calculations examples
What is the concentration of an NaOH solution if 25.0cm3
is neutralized by 23.4cm3 0.998moldm-3 HCl solution?
3. Calculate no.
moles of NaOH:
0.0234 moles HCl = 0.0234 moles NaOH
4. Calculate conc.
of NaOH: = (0.0234 × 1000) / 25.0
conc. = (moles × 1000) / volume
= 0.936moldm-3
1. Calculate no.
moles HCl:
moles = (conc. × volume) / 1000
= 0.0234
= (0.998 × 23.4) / 1000
2. Determine ratio
of NaOH to HCl:
NaOH + HCl NaCl + H2O
What are the different types of yield?
The theoretical yield is the maximum mass of
product expected from the reaction, calculated using reacting masses.
To calculate the percentage yield, the theoretical yield
and the actual yield must be calculated.
The actual yield is the mass of product that is
actually obtained from the real chemical reaction.
The percentage yield of a chemical reaction shows
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Calculating yield
The percentage yield of a reaction can be calculated using the following equation:
percentage yield = (actual yield × 100) / theoretical yield
Example: What is the percentage yield of a reaction
where the theoretical yield was 75kg but the actual
yield was 68kg?
percentage yield = (actual yield × 100) / theoretical yield
= 90.7%
What is atom economy?
Atom economy is another measure of the efficiency of a chemical reaction. It is the mass of reactants that end up as the desired product – this is calculated as a percentage.
This concept is useful to chemical industry, because it takes into account the atoms that end up in unwanted waste products as well as the yield of the reaction.
This means a process that produces several worthless
by-products could have a high yield but a low atom economy.
Reactions with a high atom economy tend to be more
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Calculating atom economy
Example: What is the atom economy of a reaction
where the actual yield was 25000 tonnes but the mass
of the reactants was 30000 tonnes?
= 83.3%
total mass of reactants
mass of desired products × 100 atom economy =
mass of desired products × 100 total mass of reactants
atom economy =
25000 × 100
30 000