1. Relative atomic mass 3 Quantitative chemistry Topic overview
The mass of individual atoms is so small that scientists use a special scale to measure the mass of atoms relative to one another.
On this scale, the mass of a 12C atom is exactly 12.
The relative atomic mass (Ar) of an element is the average mass of atoms taking into account the mass and amount of each isotope it contains on this scale.
It can be calculated as follows:
Relative atomic mass= total mass of all atoms of element total number of atoms of that element
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2. Relative formula mass (formula mass)
3 Quantitative chemistry Topic overview
Relative formula mass (formula mass)
The relative formula mass (Mr) of a substance is the sum of the relative atomic masses of all atoms shown in the formula.
Example:
The formula of water is H2O, so the relative formula mass is the sum of the relative atomic mass of two hydrogen atom (2 x 1) and one oxygen atom (1 x 16). This adds up to 18.
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3. The mole 3 Quantitative chemistry Topic overview
A mole is a measure of the amount of a substance.
One mole of a substance contains the exact same number of particles as a mole of any other substance. This is known as the Avogadro constant.
The value of the Avogadro constant is 6.02 × 1023.
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4. Conservation of mass 3 Quantitative chemistry Topic overview
During a chemical reaction atoms cannot be created or destroyed. You will have the same number of atoms before and after a reaction (but the way they are bonded together may be different).
Because of this, your equation for the chemical reaction will be balanced – i.e. the same number of atoms on both sides of the equation.
As the same atoms are present at the start and end of the reaction, mass must be conserved – i.e. the total mass of the reactants will be exactly the same as the total mass of the products. This is the law of conservation of mass.
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5. Yield and percentage yield
3 Quantitative chemistry Topic overview
Yield and percentage yield
When a chemical reaction takes place, a certain amount of product is made. This is called the yield.
However, we are rarely able to produce everything that we originally expect to in a reaction. A reaction may not completely finish or the reactants may behave differently to how we expected. Also, small amounts of the products may be lost during the experiment.
The percentage yield is the amount of product produced during a reaction compared to the maximum theoretical mass that could be produced. The higher the percentage yield, the more efficient the reaction.
Percentage yield=100 x mass of product actually made maximum theoretical mass of product
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6. Atom economy 3 Quantitative chemistry Topic overview
Chemical reactions are used to make specific products.
Sometimes a reaction takes place and only the specific product that we need is created. This means that all the atoms in the original reactants have ended up in our desired product.
Sometimes a reaction takes place and we end up with our desired product, plus some extra products. This means that only some of the atoms from the original reactants ended up in our desired product. Some ended up in the other products.
Atom economy is a way of measuring (as a percentage) how many of the atoms from the reactants have ended up in our desired product. The higher the atom economy, the more of the mass of the reactants ended up in the desired product (a more efficient reaction).
Atom economy=100 x sum of relative formula mass of desired product from equation sum of relative masses of all reactants from equation
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7. Gas volumes 3 Quantitative chemistry Topic overview
Temperature and pressure both have an affect on the volume of a gas.
BUT if the temperature and pressure of gases are the same, equal numbers of moles in gases will have exactly the same volume.
The volume of one mole of any gas at room temperature and pressure is 24 dm3.
That means two moles of any gas under the same conditions will be 48 dm3.
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8. The concentration of solutions
3 Quantitative chemistry Topic overview
The concentration of solutions
We can measure the concentration of a solution by figuring out what mass of solute (in grams) is present in the solution. We use g/dm3.
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9. The concentration of solutions
3 Quantitative chemistry Topic overview
The concentration of solutions
We can also measure the concentration of a solution by using mol/dm3.
Effectively, how many moles of solute are dissolved in each 1 dm3 of solution.
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10. Titrations 3 Quantitative chemistry Topic overview
Titrations are a very accurate way to determine the concentration of a solution.
They work by reacting the ‘unknown’ solution with a solution that you know the concentration of.
We often carry out titrations to determine the concentration of acids and alkalis.
You will do this as one of your required practical experiments.
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