Quantitative Chemistry

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The Conservation of Mass

In a chemical reaction, the total mass of the products is equal to the total mass of the reactants. This idea is called the conservation of mass.

  • Mass is conserved (kept the same) because no atoms are lost or made
  • Chemical symbol equations must always be balanced to show this i.e. there must be the same number of atoms of each element on both sides of the equation
  • For example, when solid iron reacts with copper(II) sulfate solution, a reaction takes place, producing solid copper and iron(II) sulfate solution
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Relative Formula Mass

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of all the atoms in the numbers shown in the formula. It does not have a unit.

The relative atomic masses are shown in the periodic table.

Due to the conservation of mass, the sum of the relative formula masses of all the reactant is always equal to the sum of the relative formula masses of all the products.

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Apparent Changes in Mass

Some reactions appear to involve a change in mass. This happens when reactions are carried out in a non-closed system and include a gas that can enter or leave.

  • For example, when magnesium is burned in air to produce magnesium oxide, the mass of the solid increases.
  • This is because when the magnesium is burned, it combines with oxygen from the air and the oxygen has mass.

2Mg (s) + O2 -> 2MgO (s)

  • If the mass of oxygen is included, the total mass of all the reactants is equal to the total mass of all the products
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Amount of Substance

A mole (mol) is a measure of the number of particles contained in a substance.

  • One mole of any substance contains the same number of particles - 6.02 x 10^23
  • This value is known as the Avogadro Constant
  • The mass of one mole of a substance is its relative atomic mass or relative atomic formula in grams

Example:

One mole of sodium atoms contains 6.02 x 10^23 atoms

The relative atomic mass of sodium is 23.0

One mole of sodium atoms has a mass of 23.0g

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Calculating the Amount of substance

You can calculate the amount of substance (number of moles) in a given mass of a substance using the formula:

number of moles = mass of substance (g) / atomic (or formula) mass (g/mol)

Example:

Calculate the number of moles of carbon dioxide in 33g of the compound:

no of moles = mass of substance / formula mass

                    = 33 / 44

                    = 0.75 mol

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Balanced Equations

Balanced equations:

  • show the number of moles of each product and reactant
  • can be used to calculate the mass of the reactants and products

The numbers needed to balance an equation can be calculated from the masses of the reactants and the products using moles

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Balancing Equations

Aluminium oxide can be reduced to produce aluminium:

  •  Al2O3 -> 2Al + 1 1/2 O2

Calculate the mass of aluminium oxide needed to produce 540g of aluminium:

amount of aluminium = 540/27 = 20 mol

amount of aluminium oxide required = 20/2 =10 mol

formula mass of aluminium oxide = (27x2) + (16x3) = 102

mass of aluminium oxide needed = 10 x 102 = 1020g

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Limiting Reactants

Sometimes when two chemicals react together, one chemical is completely used up during the reaction.

  • When one chemical is used up, it stops the reaction going any further. It is called the limiting reactant
  • The other chemical, which is not used up is said to be in excess
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Mole of a Gas

At room temperature and pressure, one mole of any gas takes up a volume of 24dm^3

  • At room temperature and pressure:

volume of gas = amount (mol) x 24dm^3

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Concentration

The concentration of a solution is often measure using units of mol/dm^3:

concentration of a solution = amount of substance (mol) / volume (dm^3)

Example: If 1.00 mole of solute is dissolved to form a solution that has a volume of 1.00dm^3, the solution has a concentration of 1.00 mol/dm^3

2.00dm^3 of sodium hydroxide solution contains 0.50 moles of sodium hydroxide.

Work out the concentration of the solution:

= 0.50 mol / 2.00 dm^3

= 0.25 mol/dm^3

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Carrying out a Titration

Acids and alkalis react together to form a neutral solution. 

  • Titration is an accurate technique that can be used to find out how much of an acid is needed to neutralize and alkali
  • When neutralization takes places, the hydrogen ions (h+) from the acid join with the hydroxide ions (OH-) from the alkali to form water (neutral pH)

H+(aq) + OH-(aq) --> H20(l)

  • You must use a suitable indicator in titrations
  • If you have a strong acid and strong alkali, you could use methyl orange or phenolphthalein
  • Hydrochloric acid, nitric acid, and sulfuric acid are all strong acids
  • Aqueous sodium hydroxide and aqueous potassium hydroxide are strong alkalis
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Titration Method

  • Use the pipette to measure out a known amount of the alkali
  • Add a suitable indicator
  • Place the alkali in a conical flask and place it on a white tile
  • Place the acid in a burette 
  • Take a reading of the volume of acid
  • Add the acid to the alkali, swirling the flask to thoroughly mix
  • Continue until the indicator just changes colour
  • Take a reading of the volume of acid in the burette
  • Calculate the volume of acid added

HAZARDS:

Acids and alkalis can damage the skin or eyes, so eye protection must be worn and any spillages wiped up

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Percentage Yield

Atoms are never lost or gained in a chemical reaction, however, it is not always possible to obtain the calculated amount of product:

  • If the reaction is reversible, it might not go to completion
  • Some product could be lost when it is separated from the reaction mixture
  • Some of the reactants may react in different ways to the expected reaction

The amount of product obtained is called the yield. The percentage yield can be calculated using the formula:

  • percentage yield = yield from reaction / maximum theoretical yield x 100
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