chapter 3 amount of substance

• amount of substance n is used to count the number of particles in a substance
• this is measured in moles
• the Avogadro constant is 6.02 x 10^23 mol^-1 the number of particles in each mole of carbon-12
• one mole of atoms of any element is its relative atomic mass in grams 

• molar mass is the mass per mole of a substance
• units of molar mass are g mol^-1

amount n = mass m / molar mass M

• the molecular formula is the number of atoms of each element in a molecule

• the simplest whole number ratio of each element in a compound

• relative molecular mass compares the mass of a molecule with the mass of an atom of carbon-12

• relative formula mass compares the mass of a formula unit with the mass of ana tom of carbon-12

• many coloured salts are hydrated
• water molecules are part of their crystalline structure
• the water is known as water of crystallisation
• to calculate:

calculate the amount in mol of anhydrous salt

calculate the amount in mol of water 

find the smallest whole number ratio of the water and salt

• how accurate is an experimental formula?

not all the water may have been lost when the water was driven off

• liquids and gases are measured by volume
• 1dm^3 = 1000cm^3 = 1000ml = 1 litre

• the concentration of a solution is the amount of solute in moles dissolved in each dm^3 of solution
• a 1mol dm^3 solution contains 1 mol of solute dissolved in every 1 dm^3 of solution

• n = c x v (dm^3)

or

• n = c x ( v (cm^3) / 1000 )

• A standard solution is a solution of known concentration
• Are usually prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume 

• the molar gas volume is the volume per mole of gas molecules at stated temperature and pressure
• many experiments are carried out at room temperature and pressure
• RTP is about 20c and 101kPa pressure
• At RTP 1 mole of gas molecules has a volume of 24.0dm^3 = 2400cm^3

• amount n = volume V / Molar gas volume Vm
• When V is in dm^3 molae gas volume is 24
• When V is in cm^3 molar gas volume is 24000

• molecules making up an ideal gas have:

-random motion

-Elastic collisions

-Negligible size

-No intermolecular forces

• P - pressure (pa)
• V - volume (m^3)
• N - the number of gas molecules (mol)
• R - ideal gas constant (8.31 Jmol^-1K^-1)
• To convert:

- cm^3 to m^3 - x10^-6

-dm^3 to m^3 - x10^-3

- degrees to K - +273

-kPa to Pa - x10^3

• In a balanced equation, the balancing numbers give the ratio of the amount in moles of each substance
• The ratio is called stoichiometry

• Work out the amount in moles of whatever you can
• Use the equation to work out the amount in moles of the unknown chemical
• Work out any unknown information required

• The maximum possible amount of product is the theoretical yield
• This is difficult to achieve because the reaction may have not gone to completion
• Other reactions may have taken place alongside the main reaction
• Purification of the product may result in the loss of some product
• The actual yield obtained from a reaction is usually lower than the theoretical yield

percentage yield = (actual yield /theoretical yield) x 100

• the reagent that is not in excess will be completely used up first and stop the reaction - this is the limiting reagent

• measure how well atoms have been utilised
• reactions with high atom economies:

-produce a large proportion of desired products and few unwanted waste products

-Are important for sustainability as they make the best use of natural resources

atom economy = (sum of molar masses of desired products/sum of molar masses of all products) x 100