4.4 MOLES I Flashcards
Define Relative Atomic Mass
- The weighted average mass of the isotopes of an element
- It is measured on a scale on which a carbon-12 atom has exactly 12 unit
Define Relative Molecular Mass
- The total average mass of one molecule of a compound
- Atoms which are compared to 1/12th of one atom of Carbon-12
Symbol for Relative Atomic Mass
Ar
Symbol for Relative molecular mass
Mr
How do you work out the Mr of a substance
- If a word compound, find the formulae
- Break each atom in the substance down to it’s Ar
- Add up the ammounts
Unit for Ar/Mr
No unit
What is Avogadro’s constant
6.022 x10^23 atoms/molecules
Symbol for Avogadro’s constant
NA
How to calculate the number of molecules in a certain amount of a substance
Amount of any substance (in moles) x Avogadro’s Constant
Equation for composition of substances
% of Substance = Ar of substance / Mr of Compound x100
What should be done before the equation if working out the composition of a substance that is a word formulae
Convert it into a molecular formula
What does a mole represent
How many particles of substance there are
Formulae for Moles
Moles = mass (g) / Ar or Mr
Formulae for mass (g)
Mass (g) = Moles x Mr or Ar
What must you do to a substance where the mass ISN’T in grams
Convert it FIRST
What is the empirical formulae
It gives the simplest whole number ratio of atoms
How to work out the empirical formula
- Take the mass/percentage and divide by the Ar of the element you are finding (finding the moles)
- Divide each amount by the lowest answer from the previous step
- Write as a ratio then put into formula layout
What must you never do when calculating empirical formula
Round up or down
When calculating empirical formula, can you use fractions in steps
No - Always just use decimals
What does molecular formula show
Actual number of atoms in a molecule
How to calculate molecular formula
- Calculate Mr of the substance with the empirical formula
- Divide the RFM by the Mr of the substance (previously worked out)
- Multiply the substances in the empirical formula by this number
What is water of crystallisation
When salt crystals are hydrated and the water is trapped inside
How to find the amount of water in a water of crystallisation question
- Calculate the mass of water lost (Starting mass - final mass = mass of water)
- Calculate moles of the salt (Use mass of salt ONLY and divide by Mr of salt ONLY)
- Do the same with the water
- Divide the salt value and water value by the smallest value
- Use the ratio to find the amount of water
How to find the mass of a substance needed to react with an amount of another substance (reacting masses)
- Convert mass into moles of the substance we know the mass of
- Use the ratio from the balancing numbers to find the moles of the substance we need
- Convert that into mass using mass=moles/Mr
What is a limiting substance
when there isn’t enough of a substance
What is a substance in excess
When there is more than enough for that reaction
Determine which is limiting and which is excess in : 10g of Mg and 10g HCl in Mg + 2HCl → MgCL2 + H2
- Calculate the moles of Mg
- Use the ratio from the balancing to find moles of the other substance
- Find the mass of that substance
- The HCl is limiting as 30.4g is needed whilst we only have 10
- Therefore Mg is in excess
How to work out how many moles of a product is formed when you have the moles of the products
- Balance equation
- Divide number of moles of the products by the balancing number
- Multiply the smallest value by the balancing number of the product
Why is percentage yield never %100
- Loss of substances
- Unreacted material
- Impure substances
Percentage yield equation
Percentage yield = Experimental Yield / Theoretical Yield x100
How to calculate the percentage yield if given masses
- Find a balanced equation
- Find the moles of the product
- Use the ratio of balancing numbers to then find moles of the product
- Find mass of product
- Use %yield equation with mass from calculation and from question
