Quantitative Chemistry

Relative atomic mass - RAM

This is the relative mass of the element. It's the mass number.

E.g 

Silicon = 28

Magnesium = 24

Relative Formula Mass - RFM

The relative formula mass (Mr) of a compound is the sum of relative atomic masses of the atoms in the numbers shown in the formula.

E.g

NaOH = (1 x 23 ) + (1 x 16) + (1 x 1) = 40

Percentage Mass

This tell you the pecenatge mass of an element within a compound.

To work the out percetage mass:

RAM of element / RFM of compound x 100

The mole is an amount of a substance.

One mole of a substance contains the same number of stated particles, atoms, molecules or ions as one mole of any other substance.

The number of atoms , molecules or ions in a mole of a given substance is the Avogadro Consatant. The vale for avogado constant is 6 x 10 to the power of 23.

The mass of one mole is the same as the RAM/RFM .

The symbol for the unit mole is mol.

To work out the number of particles:

1 mol = 6 x 10

To work out the number of moles :

Moles = Mass / RAM or RFM

To work out the mass from Moles , you'll have to rearrange the formula . 

To  Mass = Moles x RFM or RAM

Limiting reactants 

In a chemical reaction involving two reactants , it is common to use an excess of one of the reactants to ensure taht all of the other reactant are completely used up.

The reactant that is completely used up is called the limithing reactant because it limits the amount of products.

The amount of product formed is directly proportional to the amount of limiting factor.

One mole of any gas at a given temprerature and pressure will always have the same volume.

At room temprerature and pressure (r.t.p), one mole of any gas has a volume of 24 dm cubed(liters) / 24,000 cm cubed.

rtp = 20 degrees celsius , 1 atm

Gases and Volume Calculations

Moles = Volume / 24 or 24,000

Volume = Moles x 24 or 24,000

You will know which one to use from what the question is asking you .

Percentage yield is calulating the percentage of your desired product that was made in a reaction.

Percentage yield = (Actual yield / Predicted yield) x 100 

Most reactions almost never get 100% yield because 

1. The reaction may not go to completion because it's reversible

2. Some of the product may be lost when separating from reaction mixture

3.Some of the reactions may react differently than expected

A high atom economy means that there is minimal waste, all the atoms in a reactant have become the desired product.These chemical reactions are more sustainable and efficient.

A low atom economy means that some molecules in the reactant have been wasted. This means that this chemical reaction is less efficient and less sustainable.

Atom Economy

(RFM of desired product / RFM of all product ) x 100

Concentration = (Number of moles x 1000) / Volume

Rearranging the formula

 Moles = (Concentration x Volume) / 1000

 Volume = ( Moles x 1000) / Concentration

Why are Titration repeated ?

To make the reading more accurate and therefore more reliable.

Large differences in readigs, indicates the method was not accurate.

Small differences are normal, so an average is taken .