1.2 Amount Of Substance Flashcards
How do we calculate empirical formula from mass?
Eg Determine the empirical formula of a compound that contains 2.72 g of carbon and 7.28 g of oxygen.
So, the empirical formula of the compound is CO2
How do we calculate empirical formula from Percentage?
Eg Determine the empirical formula of a hydrocarbon that contains 90.0% carbon and 10.0% hydrogen.
So, the empirical formula of the compound is C3H4
How can we calculate molecular formula from the Empirical Formula?
- The molecular formula can be found by dividing the relative formula mass of the molecular formula by the relative formula mass of the empirical formula
- Multiply the number of each element present in the empirical formula by this number to find the molecular formula
Formulae of Ionic Compounds Table:
What are Polyatomic ions?
Ions that contain more than one type of element, such as OH-
Why may reaction yield not be 100%
- Other reactions take place simultaneously
- The reaction does not go to completion
- Reactants or products are lost to the atmosphere
Percentage Yield Equation:
The percentage yield shows how much of a particular product you get from the reactants compared to the maximum theoretical amount that you can get:
Where actual yield is the number of moles or mass of product obtained experimentally
The predicted yield is the number of moles or mass obtained by calculation
Mole Definition:
The mole is the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12.
Relative Atomic Mass Definition:
- Relative atomic mass is the average mass of one atom
- compared to one twelfth of the mass of one atom of carbon-12
Relative Molecular Mass Definition:
- Relative molecular mass is the average mass of a molecule
- compared to one twelfth of the mass of one atom of carbon-12
What is Avogadro’s Number?
There are 6.022 x 1023 atoms in 12 grams of carbon-12. Therefore explained in simpler terms ‘One mole of any specified entity contains 6.022 x 1023 of that entity’:
How do we calculate moles for For pure solids, liquids and gases?
How do we calculate moles for Gases?
OR Moles = Volume/24
How do we calculate moles for Solutions?
Moles = Concentration x Volume
Unit of concentration: mol dm-3 or MUnit of volume: dm3
Converting volumes:
How do we convert between C (Degrees Celcius) and K ?
How do we calculate Number of atoms / Particles?
How do we calculate Density?
Calculating the Number of Particles Example:
Calculating Density Example:
Calculating Empirical Formula Example:
How can we calculate the Mr of a substance from a mass spectrometer?
The molecular ion ( the peak with highest m/z) will be equal to the Mr.
Calculating Concentration Examples
How do we convert concentration measured in mol dm-3 into concentration measured in g dm-3?
How do we convert from dm3 to m3?
Divide by 1000
How do we convert from cm3 to m3?
Divide by 1 Million
What is the Ideal gas equation (including units)?
Temperature (K) - Kelvin
(C > K = +273)
Ideal gas constant given in exam
What assumptions are made in the Ideal Gas equation?
(The Kinetic Theory of Gases)
- We assume that the gas molecules are tiny, compared to the spaces between them
- We also assumes that there are no forces acting between the gas molecules
- We assume that the gas molecules move randomly
- We assume that when the gas molecules collide, the collisions are elastic
- That gas molecules are moving very fast and randomly
- That molecules hardly have any volume
- The temperature of the gas is related to the average kinetic energy of the molecules
There is no such thing as an ideal gas
Example using the ideal gas equation:
How do we calculate Percentage Yield?
Calculating Percentage Yield Example:
Why do chemists want a high percentage yield?
Chemists want a high percentage yield as means there has been an efficient conversion of reactants to products
How do we calculate Percentage Atom Economy?
Do use balancing numbers when calculating % atom economy
Why do chemists want a high percentage atom economy?
Chemists want a high percentage atom economy so that the maximum mass of reactants ends up in the desired product (so minimising the amount of by-product).
What is Water of Crystalisation?
- Water of crystallisation is when some compounds can form crystals which have water as part of their structure
- A compound that contains water of crystallisation is called a hydrated compound
The water of crystallisation is separated from the main formula by a dot when writing the chemical formula of hydrated compounds
E.g. hydrated copper(II) sulfate is CuSO4∙5H2O
What do we call a compound which doesn’t contain water of crystallisation?
An anhydrous compound
E.g. anhydrous copper(II) sulfate is CuSO4
How can we convert hydrated compounds to anhydrous compounds?
By heating
How can we calculate the degree of hydration by heating?
- The mass of the hydrated salt must be measured before heating
- The salt is then heated until it reaches a constant mass
- The two mass values can be used to calculate the number of moles of water in the hydrated salt - known as the water of crystallisation
Example:
What does the volume of an Ideal Gas depend on?
- Its pressure
- Its temperature
How does the Temperature affect the volume of an Ideal Gas?
- When a gas is heated (at constant pressure) the particles gain more kinetic energy and undergo more frequent collisions with the container wall
- To keep the pressure constant, the molecules must get further apart and therefore the volume increases
- The volume is therefore directly proportional to the temperature (at constant pressure)
What are the Limitations of the Ideal Gas Law:
At very low temperatures and high pressures real gases do not obey the kinetic theory as under these conditions:
- Molecules are close to each other
- There are instantaneous dipole- induced dipole or permanent dipole- permanent dipole forces between the molecules
- These attractive forces pull the molecules away from the container wall
- The volume of the molecules is not negligible
Units for the Ideal Gas Equation:
P = pressure (pascals, Pa)
V = volume (m3)
n = number of moles of gas (mol)
R = gas constant (8.31 J K-1 mol-1)
T = temperature (kelvin, K)
How do we convert from Degrees to Kelvin?
+ 273
