Chapter 3 Amount Of Substance Flashcards
Avogadro’s Constant (NA)
6.02*10^23 mol^-1 ; number of particles in each mole of carbon 12
Mass of 1 mol of atoms of any element
Equals to the relative atomic mass in grams
1 mol of Carbon
12g
1 mol of Hydrogen
1g
Define the mole
The amount of any substance containing as many particles as there are carbon atoms in exactly 12g of carbon-12
Molar mass (M)
Gives the mass in grams in each mole of the substance - units are g mol^-1 (mass/mol)
Mol equation
Amount (mol - n) = mass (m - g) / molar mass (M - g mol^-1)
What is a molecule?
Two or more atoms held together by covalent bonds
Molecular formula
Number of atoms of each element in a molecule
Elements that exist as molecules
H2, N2, O2, F2, Cl2, Br2, I2, P4 and S8
Empirical formula
Simplest whole number ratio of atoms of each element in a compound
Why is empirical formula important?
It helps in simplifying elements that do not exist as molecules (ionic/non-metals) - where giant lattices are formed with billions of atoms so this shows the simplest ratio that never changes
Relative molecular mass
The mass of a molecule relative to the mass of an atom of carbon 12 - ONLY FOR SIMPLE MOLECULES
Relative formula mass
Compares the mass of a formula unit with the mass of an atom of carbon 12 ; INLY EMPIRICAL FORMULA (ionic lattices etc)
What is analysis?
Investigating the chemical composition of a substance is called analysis - results of an experiment
How to work out empirical formula from mass?
CONVERT TO MOLES AND THEN SIMPLEST RATIO (DIVIDE BY SMALLEST NUMBER - DO NOT ALWAYS ROUND AND SEE WHAT RATIO IS POSSIBLE)
How to determine molecular mass?
Percentage composition/molar mass (replace mass (g) with percentage in equation) and then empirical formula divided by total molar mass
Multiply whole empirical by factor to give MOLECULAR FORMULA
Hydrated salts
Water part of crystalline structure - water of crystallisation is what makes CuSO4.H2O blue and when heated bonds are broken and white anhydrous CuSO4 left behind
Formula of hydrated copper sulfate to anhydrous
CuSO4.5H2O -> CuSO4 + 5H2O
Experiment to find out formula of hydrated salt
Weigh an empty crucible
Weigh crucible + hydrated salt
Heat the crucible gently using a Bunsen Bruner clay pipe triangle and tripod - heat to a constant mass till consecutive values are the same (make sure not to decompose)
Weigh mass of crucible + anhydrous after cooling
ASSUMPTIONS when heating hydrated salt
1) Not always going to be a distinct colour change therefore must heat to a constant mass - crystals reheated until the mass of residue does not change ; ALL water removed ; if not then value of x is smaller than true value
2) Many salts may further decompose when heated so blue CuSO4 may turn to black CuSO4 - then value of x higher than real value
1cm^3 into ml
1 ml
1dm^3 into ml
1000ml = 1000cm^3 = 1 litre
Cm3 to dm3
Divide by 1000
Equation linking moles and volume and concentration
n= c*v
What are standard solutions and how are they prepared?
Known concentration - dissolve an exact mass of solute in a solvent and make up the solution to an exact volume => INVERT FLASK SLOWLY AND TAKE IN ALL THE WASHINGS
How to convert from mol/dm^3 to g/dm^3 or vice versa?
Use mass = mr*moles so moles = mass/mr
What about gases - how to measure the volume of gases?
Measure gas volumes - at same temperature and pressure equal volumes of different gases contain the same number of molecules
What is molar gas volume?
Volume per mole of gas molecules at a stated temperature and pressure
RTP
20 degrees Celsius and 101000 Pa (1 atm)
Molar gas volume at RTP
24 dm^3/mol
Volume of a gas at RTP Equation
V = n*24dm^3/mol
Why do we use ideal gas equation?
RTP will be approximate so when gases are at different temperatures or you need to be accurate use the ideal gas equation
Assumptions for molecules in an ideal gas
- No intermolecular forces between particles
- All collisions are perfectly elastic (no KE lost)
- Temperature is proportional to KE
- Moving in random motion in straight lines
Ideal gas equation
pV = nRT
Pressure
Measured in Pa
Volume (V)
Measured in m^3
Cm^3 to m^3
*10^-6
n
Moles (mass/mr)
R
Constant of 8.314 J/mol/K
T
Temperature in Kelvin +273 TO GET KELVIN
Experiment to find RFM of a liquid
Must be liquid at RTP but boiling point below 100
1) Weigh mass of gas syringe
2) Inject sample info a gas syringe through the self-sealing rubber cap and reading the to find mass of volatile liquid added
3) Place in boiling water bath at 100 and the liquid vaporises producing a gas with atmospheric pressure and volume of gas in syringe recorded
Stoichiometry
Ratio of the amount in moles of each substance (balancing numbers)
What do we use balanced equations for?
Quantities of reactants required to prepare a required quantity
Quantities of products that should be formed from certain quantities of reactants
Experiment to identify an unknown metal
1) Conical flask containing acid + metal attached to gas syringe
2) Weigh sample of metal and add to acid
3) Using measuring cylinder add standard solution (in excess) of acid and put on bung
4) Measure maximum volume of gas created in syringe
Standard solution of acid
25 cm^3 = 0.025 dm^3
1 moldm^-3
Theoretical yield
The maximum possible amount of product created (assumption) - all of the reactants turned into products
Why is maximum theoretical yield not always achieved?
Reaction may not have gone to completion
Side reactions may have taken place alongside main reaction
Purification/filtration may lead to loss of product
Actual yield
The actual amount of product created - usually less than theoretical yield
Percentage yield
Conversion of starting materials into a desired product
Actual yield/theoretical yield * 100
Limiting reagent
Reactant not in excess that is completely used up first and stops the reaction
How to find out limiting reagent?
Compare available moles with the ratio (stoichiometry - balanced equation)
2H2 + O2 -> 2H2O
If equal amounts of hydrogen and oxygen, hydrogen will be used up first because 2:1 ratio and half of the oxygen would be unreacted (excess)
Atom economy
(Sum of molar masses of desired / sum of molar masses of all products) * 100
High atom economy means…
A large proportion of products are desired (few waste)
Good sustainability because atoms well utilised and good use of natural resources
Atom economy problem
ASSUMES 100% YIELD - ALL REACTANTS ARE CONVERTED TO PRODUCTS
Improving atom economy
Helps in preserving finite resources and prevents disposal of waste - improves efficiency and in an ideal chemical process a use would be found for all products (thus atom economy would be 100%)
What else should be taken into account when judging sustainability?
The availability of reactants (starting materials may be low or not - affects prices)
Efficiency depends on percentage yield too
