Quantitive Flashcards
Relative formula mass (Mr)
The sum of the relative atomic masses of all the atoms present in the formula of a substance
Equation for % element in a compound (% composition)
% element A = Ar or A x number of atoms of A in compound
—————————————————————— X100
Mr of whole compound
Mole
One of a substance contains 6.02x10^23 atoms/molecules of that substance
Molar mass
The mass of one mole of the substance and is numerically equal to the RAM/RFM in grams
Mole equation
Mass (g)
—————
Molar mass
Actual yield
The mass of the product measured at the end of the reaction
Theoretical yield
The max yield which can be produced using the calculation using a balanced chemical equation for the reaction
% yield
Actual yield
———————- x 100
Theoretical yield
Limiting reactant
The reactant which runs out first in a reaction, limiting the amount of product that can be made
Empirical formula
The simplest whole number ratio of the atoms of each element in compound
Molecular formula
The actual number of atoms in each element in a molecule
Water of crystallisation
Water that is chemically bonded into the crystal structure
Anhydrous hydrated dehydration
Doesn’t contain water of crystallisation
Solid crystals contain water of crystallisation
Removal of water of crystallisation
heat to constant mass
The process of heating and weighing is repeated until the mass stops changing. This ensures that the reaction is complete
Degree of hydration
The number of moles of water of crystallisation chemically bonded in 1 mole of the compound
What is a weighted mean , why is it used to calculate Ar
It takes into account the isotopes of that element and their contribution to the overall mass of the element
Relative atomic mass units
NO UNITS
What is 1 mole of any substance and what is it known as
6.02x10^23 , avogadro’s constant
What do we do after weighing out the Mr in grams
We have 1 mole of the particle represented by that formula , therefore we can count particles by weighing them
What do you do if you’re asked to find what 1 mole of a substance weighs , molar mass
Find it’s Mr , stick a “g” sign after it
1 tonne in grams
1,000,000g
1kg in grams
1000g
Calculating reacting masses using chemical equations
Find moles
Move them using ratio
Find mass that is asked for
What do we do to work out the limiting reactant
What are the other reactant described as
We use the mole ratio from the balanced chemical equation
Being in excess
Calculations involving a limiting reactant
What will be given
Method
Masses of more than 1 reactant
Work out which is in excess and then use the other , to calculate mass of product
Calculate moles of each substance
Ratio them
Find the smallest
Convert to mass
What does % yield tell us
Gives a measure of how efficient the chemical process is
Why is % yield never 100%
Competing side reactions
This is due to other chemical reaction that may also take place
Physical loss of product in separation from the reaction mixture eg:
- loss in transfer from 1 piece of apparatus to another
- some product may be left on filter paper during filtration
- some product may stay in solution after crystallisation
Reaction is reversible and doesn’t go to completion
Empirical formula by experiment
The word empirical = determined by experiment , by determining the empirical formula of a wide range of compounds scientists came up with the rules about VALENCY that we now use to work out formula
Empirical formula working out
X Y Z
Mass
Mr
Moles
Ratio
How to find molecular formula hydrocarbon , Mr = 42 , EP= CH2
From empirical formula if you know the Mr of the compound
Add up the atomic masses of the atoms in empirical formula
The mass of the atom in the empirical formula is eg 14
42/14 = 3
So you need to multiply the numbers in the empirical by 3 therefore hydrocarbon is C3H6
Finding the formula of copper oxide
What does the hydrogen do
How to work out the mass of O2 in CuO
What can be calculated from these values
Passed over heated copper oxide it reduces it, taking away oxygen to leave copper
By measuring the initial mass of the copper oxide and the final mass of copper obtained
Formula of copper oxide
Finding out the formula of copper oxide
Observations
HOW DOES ALL THE COPPER OXIDE BEEN REDUCED
Black solid (CuO) turns red-brown
Solid condensation initially appears on the cooler part of the tube
After heating apparatus must be allowed to cool down before the supply of hydrogen gas is turned off as the hot metal would reoxidise into copper oxide
HEATING TO A CONSTANT MASS , this is the process of heating and weighing until the mass stops changing, ensures that the reaction is complete
Finding the empirical formula of MgO
Cut a piece of magnesium about 5 cm long , if looks black or tarnished then clean it using emery paper, twist it into a loose coil
Weigh the crucible with the lid and then the magnesium inside the crucible with the lid
Light the bunsen and begin to heat the crucible
Once the crucible is hot gently lift the lid using tongs a little, to allow some oxygen to get in you may see the magnesium begin to flare up
(If the lid is off for to long then the magnesium oxide product will begin to escape don’t let this happen)
Keep heating and lifting with the lid until you see no further chemical reaction
At this point remove the lid and heat for another couple of minutes , replace the lid if it appears you are loosing some product
Turn off the Bunsen and allow apparatus to cool
Re-weigh the crucible with the lid containing the product
Heat the crucible again for another couple of minutes and allow to cool again
Repeat this step until the mass reading is consistent called “heat to a constant mass”
Test for water
When water is added to anhydrous copper(II) sulfate it turns from a white powder to blue
Degree of hydration
CuSO4.5H2O
The 5
The number of moles of water of crystallisation chemically bonded in 1 mole of the compound
Calculating Mr and the % of water for hydrated salts
What do some ionic compounds contain
Eg CuSO4.5H2O contains,……… water molecules
Mr = 250
Water of crystallisation, this must also be counted when calculating Mr
5
%water = 90/250 x100
=36%
Working out the formula of a compound containing water of crystallisation experimental results
What to do
Take away hydrate from anhydrous to get H2Og and anhydrous is the other g
X H2O
g
Mr
Moles
Ratio
Formula
Determining the formula of a compound containing water of crystallisation experimentally
How to calculate
1 the mass of the anhydrous salt
2 the mass of the water in the crystals
We need to take a sample of hydrated salt and heat to drive off the water of crystallisation. In the experiment we need to find 1 and 2
The residue left after heating
The decrease in mass on heating
Finding the mass of water in a hydrated compound
Weigh and record mass of crucible
Add around 25g of hydrated compound, weigh and record mass of crucible and hydrated compound
Place the crucible in a pipe clay triangle and heat to remove the water of crystallisation
Allow the crucible to cool, then weigh and record mass
Repeat the heating, cooling a and reweighing until two masses are the same (heat to constant mass)
Finding the formula of hydrated CuSO4
Weigh empty crucible
Weigh into it 2-3g of hydrated CuSO4 , record all weighings accurate to the nearest 0.01g
Set up apparatus as shown
Heat the crucible and contents gently at first,over a medium Bunsen flame so that the water of crystallisation is drove of steadily . The blue colour of hydrated compound should gradually fade into an anhydrous greyish white.
Avoid over heating which may cause further decomposition and stop heating , toxic or corrosive fumes may be evoked.
A total heating time of about 10minutes should be enough
Allow crucible and contents to cool, the tongs may be used to remove the crucible from the hot clay triangle onto the heat resistant mat where it should cool more quickly
Re-weigh the crucible and its contents once cold
