EL6 Amount Of Substance Flashcards
Define relative atomic mass
Mass of atom relative to carbon 12
Define relative formula mass (molecular mass)
Sum of relative atomic masses of elements making up compound
How is number of moles calculated?
Mass divided by relative formula mass
Define empirical formula
Simplest whole number ratio of atoms of each element in a compound
Define molecular formula
Actual number of atoms of each element in a molecule
What is water of crystallisation?
- water molecules are fitted within some ionic lattices
- when crystals are heated, water of crystallisation is removed as steam leaving anhydrous solid
Why is experimental yield always lower than expected yield in an experiment?
- loss of products
- side reactions occurring
- impurities in reactants
- changes in temp and pressure
- if reaction is in equilibrium system
What is the equation for percentage yield?
Experimental yield divided by Theoretical yield X100
Calculate the number of moles in 4g of oxygen
4g / 16g mol^-1 = 0.25 mol
Calculate the relative formula mass for methane
(1 x 12) + (4 x 1) = 16
Calculate the chemical formula from the moles when 0.84g of magnesium is reacted with 0.56g of oxygen.
Mg: 0.84 g / 24 g mol^-1 = 0.035 mol
O: 0.56 g / 16 g mol^-1 = 0.035 mol
The reaction is a 1:1 ratio so therefore MgO is the formula
Methane is 75% carbon by mass and 25% hydrogen by mass. Work out the formula for methane
C: 75 g / 12 g mol^-1 = 6.25 mol
H: 25 g / 1 g mol^-1 = 25 mol
Divide both by the smallest value calculated.
C: 6.25 / 6.25 = 1
H: 25 / 6.25 = 4
Therefore answer is CH4
Calculate the formula of the hydrated compound of MgCl2.XH2O when is goes from 2.53 g to 1.17 g during heating
Mass of water lost = 2.53 - 1.17 = 1.36g
Moles of water lost = 1.36 g / 18 g mol^-1 = 0.076 moles
Anhydrous MgCl2 = 1.17 g
Moles of anhydrous MgCl2 = 1.17 / 95.9 g mol^-1 = 0.012 moles
Divide both by the smallest calculated mole value to give:
MgCl2: 0.012 / 0.012 = 1
H2O: 0.076 / 0.012 = 6.3
Round to 6 to give MgCl2.6H2O
X = 6
