Formulae and Equations Flashcards
what is the molecular formula
formula that shows the number and type of each atom in a molecule
what is empirical formula
simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound
how to work out molecular formula
calculate empirical mass
divide relative formula mass by empirical mass
Multiply each number of elements by this number
water of crystallisation
when some compounds can form crystals which have water as part of their structure this is called a hydrated compound.
PV = nRT
p = (Pa)
V = (m3)
n = (mol)
R = 8.31 Jmol-1K-1
T = (K)
what is a precipitation reaction
reaction of two aqueous solutions to form products which include one solid
what is avogadros constant
number of particles equivalent to the relative atomic mass or molecular mass of a substance
moles from PV = nRT
n = PV/RT
how to work out number of moles with molar gas volume
volume / molar gas volume
molar gas volume in room and standard temp
at room temp it is 24.0
at standard it is 22.4
what is volumetric analysis
process that uses the volume and concentration of one chemical reactant (standard solution) to determine the concentration of another unknown solution
how can you measure volumes
volumetric or graduated pipette and a burette
how can you make a standard solution
Weigh the sample bottle containing the solid on a (2 dp) balance.
Transfer solid to beaker and reweigh sample bottle.
Record the difference in mass.
Add distilled water and stir with a glass rod until all the solid has dissolved.
Transfer to a volumetric flask with washings.
Make up to the 250cm° mark with distilled water.
Shake flask.
what are common errors when making volumetric solution
Common errors in this method include systematic errors on the balance, lost substance in transfer processes and overfilling of the volumetric flask. These can be reduced using washing methods and by reading volumes from the bottom of the meniscus.
how to preform the titration
Measuring a known volume (usually 20 or 25 cm3) of one of the solutions with a volumetric pipette and placing it into a conical flask
The other solution is placed in the burette
To start with, the burette will usually be filled to 0.00 cm3
A few drops of the indicator are added to the solution in the conical flask
The tap on the burette is carefully opened and the solution added, portion by portion, to the conical flask until the indicator starts to change colour
As you start getting near to the end point, the flow of the burette should be slowed right down so that the solution is added dropwise
You should be able to close the tap on the burette after one drop has caused the colour change
Multiple runs are carried out until concordant results are obtained
Concordant results are within 0.1 cm3 of each other
uncertainty in burettes
Burettes are usually marked to a precision of 0.10 cm3
Since they are analogue instruments, the uncertainty is recorded to half the smallest marking, in other words to ±0.05 cm3
what is the concentration of a solution
amount of solute dissolved in a solvent to make 1 dm3 of solution
what is a dilute concentration
solution with a low concentration of solute
what can affect a random error
How easily the instrument or scale is to read
The person reading the scale poorly
Changes in the environment
what is a systematic error
errors that occur as a result of a faulty or poorly designed experimental procedure
examples of systematic error
forget to zero an electronic balance (using the tare button) the mass weighings will always be higher than they should be
If you don’t read the volume in a burette at eye level, the volumes will always be smaller than they should be due to a parallax error
formula for percentage uncertainty
absolute uncertainty / measured value x 100
what happens when you multiply or divide experimental measurement
you add together percentage uncertainties and then calculate absolute uncertainty from sum of percentage uncertainties
what happens when you add or subtract two measurements
you add together the absolute measurement uncertainties
in reactions why do not all reactants react to form products
Other reactions take place simultaneously
The reaction does not go to completion
Products are lost during separation and purification
how can you work out percentage yield
actual yield / theoretical yield x 100
how can you work out actual yield and theoretical yield
number of moles or mass of product obtained experimentally
number of moles or mass obtained by a reacting mass calculation
how can you work out actual yield and theoretical yield
number of moles or mass of product obtained experimentally
number of moles or mass obtained by a reacting mass calculation
what is atom economy
how many of the atoms used in the reaction become the desired product
atom economy formula
molecular mass of desired / sum of molecular mass of all reactants x 100
what is meant by concordant results
titration results that are within 0.2 (cm3) of each other
what is meant by molar volume of a gas
volume space occupied by one mole of a gas at a specified temperature and pressure/rtp/st/standard conditions
