Atomic strucure, analysis & quantitive chemistry


Mass Number & Atomic Number

Atoms of an element can be described using their mass number & atomic number

The mass number is the total number of protons & neutrons in the atom 

the atomic (proton) number is the number of protons in the atom

the number of protons in an atom is equal to the number of electrons, so an atom has no overall charge 

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All atoms of a particualr element have the same number of protons. Atoms of different elements have different numbers of protons

Isotopes are atoms of the same element that have different numbers of neutrons

isotopes have the same atomic number but a different mann number 

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Relative Atomic Mass, Ar

the relative atomic mass, Ar, of an element is found on the periodic table. it is the larger number shown for each element. 

the relative atomic mass, Ar, is the mass of a particular atom, compared with a twelth of the mass of a carbon atom. 

the Ar, is an average value for all the isotopes of an element 

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Relative Formula Mass, Mr

the relative formula mass Mr, of a compound is the relative atomic masses of al its elements added together. 

To calculate Mr, you need to know:

  • the formula of the compound
  • the Ar of all the atoms involved 
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Calculating Percentage Mass

the mass of the compound is its relative formula mass in gras. 

to calculate the percentage mass of an element in a compound, you need to know:

  • the formula of the compound
  • the relative atomic mass of all the atoms 

you can calculate the percentage by using the formula:

relative mass of element in compound/relative formula mass of comound x 100 

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Empirical Formula of a Compound

the empirical formula of a compound is the simplest whole number ration of each kind of atom in the compound. 

1. identify the mass of the elements in the compound

2. divide these masses by their relative atomic masses

3. identify the ratio of atoms in the compound 

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The Mole

A mole is a measure of the number of particles contained in a substance. One mole of a substance is its relative formula mass or Ar in grams

one mole of any substance will always contain the same number of particles - 6x10^23, this is the relative formula mass of the substance

if a substance is an element, the mass of one mole of the substance, called the molar mass is always equal to the relative atomic mass of the substance in grams. 

if a substance is a compound, the mass of one mole of the substance is always equal to the relative formula mass of the substance in grams 

you can calculate the number of moles in a substance using this formula:

number of moles of substances = mass of substance / mass of one mole

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Calculating the Mass of a Product

1. Write down the equation

2. Work out the Mr of each substance

3. Check that the total mass of reactants equals the total mass of the products.

4. You need the ration of mass of reactant to mass of product

5. Use the ratio to calculate how much can be produced

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Instrumental Methods

Standard laboratory equipment can b used to detect & identify elements & compounds. Instrumental methods that involve using highlt accurate instruments to analyse & identify substances have been developed to perform this function.these instruments give rapid results, r very sensitive & accurate & can be used on small samplesan example of a common instrumental method is gas chromotography linked to mass spectroscopy (GC-MS)

A GC-MS works by allowing different substances, carried by a gas, to travel through a column packed w solid material at different speeds so that they seperate out. each substance will produce a seperate peak on an output known as a gas chromotograph

the number of peaks on this output shows the number of compounds present in the original sample

the positiion of the peaks on the output graph indicates the retention time 

if the output of the gas chromotography column is linked to a mass spectrometer then this can also be used to identify the substances leaving the column

the mass spectrometer can give the relative molecular mass of each substance seperated in the column. the molecular mass is gived by the molecular ion peak on the spectrum

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Chemical analysis can be used to identify additives in food. Chromotography is used to identify articficial colours, by comparing them to known substances. 

1, samples of 5 known food colourings, and the unknown substance are put on a 'start line' on a piece of paper

2. the paper is dipped into a solvent, the solvent dissoles the samples & carries them up the paper 

3. substance x can be identified by comparing the horizontal spots

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Atoms are never lost or gained in a chemical reaction. but, its not always possible to obtain the calculate amount of the product for several reasons:

  • if the reaction is reversible, it might not go to completion
  • some product could be lost when its seperated 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 & yield can be calculated by comparing:

  • the actual yield obtained from a reaction
  • the maximum theoretical yield 
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