3. Stoichiometry Flashcards
Ions
A charged particle produced by the loss/gain of electrons.
What sort of ions do metals form?
Positively charged ones.
atoms lose electrons
What sort of ions to non-metals form?
Negatively charged ones.
atoms gain electrons
Ionic Bonds
The electrostatic force of attraction between positively and negatively charged ions.
Relative Atomic Mass
The average mass of the atoms of an element compared with carbon 12.
Relative Formula Mass
The total of the relative atomic masses, added up in the ratio of the chemical formula of a substance.
Mole
The amount of substance in the relative atomic or formula mass in grams.
No of Moles =
Mass (g) / Ar or Mr
To calculate the percentage of an element in a compound:
> Work out the Mr of your compound, writing down the Ar of each element separately as you go.
Divide the Ar of your element by the total Mr of the compound.
x 100
To find an empirical formula:
> Divide the amount of each element by its Ar.
Put what you get into a ratio.
Divide all of them by the smallest no. in the ratio.
Make a formula.
Total Mass of Products =
Total Mass of Reactants
Aqueous Solutions
Mixture made by adding a soluble substance to water.
Litmus + Acid
Red
Litmus + Alkali
Blue
Percentage yield of a chemical reaction =
amount of product produced/ maximum amount of product possible (x100)
Why might a percentage yield be less than it should be?
> Reaction might be reversible
May have given unexpected products in alternative reactions.
Some might be lost whilst handling apparatus.
Lost during separation from reaction mixture.
What is relative atomic mass (Ar)?
average mass of naturally occurring atoms of an element on a scale where the 12C atom has a mass of exactly 12 units
What is relative molecular mass? (Mr)
sum of the relative atomic masses
What is relative formula mass?
Relative molecular mass for ionic compounds
How can solution concentrations be expressed?
mol/dm3 or g/dm3
Moles =
Concentration x Volume
What is a mole?
the number of particles which is equal to the number of atoms in 12g of carbon 12
What is Avogadro’s constant? (3)
number of particles / atoms / ions / molecules in one mole of a substance
the number of particles / molecules in 24dm3 of a gas at RTP
What is the number of particles in 24 dm3 of a gas at RTP?
6 to 6.0^23 × 10^23 atoms (Avogadro’s constant)
no. of moles =
mass of substance taken/mass of one mole of substance
What is the limiting reactant?
the one that is not in excess
How can we calculate the limiting reactant?
Calculate which reactant has the lower number of moles
Excess of substance =
moles of substance at start - moles of substance at end
n =
M/mr
concentration =
number of moles / volume
volume of gas (in dm3) =
number of moles x 24
% by mass of element in compound =
sum of Mr/ relative formula mass of compound x 100
% yield =
actual yield/predicted yield x 100
% purity =
mass of pure product/mass of impure product x 100
Define the mole
The mass of substance containing the sane number of fundamental units as there are atoms in exactly 12g of 12C
What is Avogadro Number?
One mole of a substance contains 6.02 * 10 ^23
What is the molar volume?
24dm^3 at room temperature
What are the calculations involving gases?
Amount of gas (mol) = Volume of gas (dm^3) /24
Amount of gas (mol) = Volume of gas (cm^3) /24000
What is the main equation for calculating masses, moles and solutions?
Moles= mass of substance/ Relative formula mass
What is the equation to calculate concentration?
Concentration (mol/dm^3) = Amount of substance(mol)/
Volume of solution(dm^3)
What is the empirical formula?
Gives the simplest whole number ratio of atoms of each element
What is the molecular formula
Gives the exact numbers of atoms of each element present in the formula of the compound
What is the equation used to find the percentage yield?
Yield obtained/theoretical yield * 100
What is the equation used to find the percentage purity?
Percentage purity= Mass of pure substance/mass of impure substance *100
Define relative atomic mass
The average mass of naturally occurring atoms of an element on a scale where the 12C atom has a mass of exactly 12 units
Define relative molecular mass
The sum of the relative atomic masses
Define the molecular formula of a compound
the number and type of different atoms in one
molecule
Define the empirical formula of a compound
the simplest whole number ratio of the different
atoms or ions in a compound
Describe relative atomic mass, Ar
the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of 12C
Describe relative atomic mass, Ar
the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of 12C
Define relative molecular mass, Mr
the sum of the relative atomic masses. Relative formula
mass, Mr, will be used for ionic compounds
what is Avogadro’s constant?
6 x 10 ^23
Valency or combining power tells you how many bonds an atom can make with another atom. What is the valency for the following groups:
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
I = 1 valency.
II = 2 valencies.
III = 3 valencies.
IV = 4 valencies.
V = 3 valencies.
VI = 2 valencies.
VII = 1 valency.
VIII = 0 valencies.
For example, SO2 is called sulfur dioxide. Why?
It is called sulfur dioxide because there are two oxygen atoms.
For example, N2O4 is called dinitrogen tetroxide. Why?
It is called dinitrogen because there are two nitrogen atoms. Tetroxide is called this because it have four oxygen atoms.
What is the symbol for relative atomic mass?
The symbol for relative atomic mass is Ar.
What is the relative atomic mass (Ar)?
The average mass of naturally occurring atoms of an element on a scale where the carbon-12 atom has a mass of exactly 12 units.
NOTE - The relative atomic mass for each element can be found in the periodic table along with the atomic number (The larger number is the relative atomic mass of an element).
What is the symbol for relative molecular mass?
The symbol for relative molecular mass is Mr.
How do you calculate the relative molecular mass of a substance?
To calculate the Mr of a substance, you have to add up the relative atomic masses of all the atoms present in the formula.
What are the state symbols for the following states:
Gas.
Liquid.
Aqueous.
Concentrated.
Dilute.
Solid.
Gas = g.
Liquid = l.
Aqueous = aq.
Concentrated = conc.
Dilute = dil.
Solid = s.
Percentage yield = ?
Percentage yield = (actual yield / theoretical yield) x 100
Percentage purity = ?
Percentage purity = (pure product / impure product) x 100
Moles = ? (Solids)
Units?
Moles = Mass / Molar mass
Moles = mol
Mass = g
Molar mass(Mr) = g/mol
Describe an experiment where you have to analyse Epsom salts
MAKING HYDRATED MAGNESIUM SULPHATE CRYSTALS (Epsom salts)
1-Measure out specified volume of your H2SO4 and mass of magnesium carbonate
2-Add the spatulas of magnesium carbonate into the acid. Use a glass rod to stir the mixture until all visible signs of reaction have stopped. Do this is a beaker
3-Filter the solution using filter paper. Catch the filtrate into an evaporating basin
4-Heat the filtrate (not until it becomes dry but until its volume has reduced to about one third of its original volume
5-Leave for liquid to evaporate and crystalize
Steps of the mole calculations?
1-Balanced symbol equation
2-Reacting ratios
3-Use imformation given to you from the question to work out the moles of one of the reactants/products
4-Molar ratio
5-Moles = mass / molar mass
Work out the unknown
How to calculate empirical formulae?
1 - Use mass or % data
2 - Divide the mass/% data by the Mr of the element e.g. 12g of Sodium would mean you would so 12 / 23 = 0.56
3 - Divide the answers you get by the smallest answer
4 - Scale up or down accordingly to find the simplest WHOLE ratio
Define empirical formulae
Simplest fixed ratio of atoms present in a compound
Define molecular formulae
The actual formula of a compound e.g. the fixed ratio of atoms present in a compound
How do you work out the relative formula, Mr?
Just add all the atomic masses of the atoms it contains together
How would you work out the relative formula of MgCl2?
Mg atomic mass = 24
Cl atomic mass = 35.5
24 + (2 x 35.5) = 95
So the Mr for MgCl2 = 95
What is the term a “mole” used to represent?
The amount in a substance - convenient way of counting atoms
What does a mole allow chemists to do?
Make predictions about masses of different substances involved in a reaction
What will one mole of atoms have a mass in grams equal to?
The relative formula mass (Ar/Mr) for the substance
What does one mole weigh exactly from the following?
Carbon - Ar of 12
Nitrogen gas - Ar of 28
Carbon dioxide - Ar of 44
Carbon - 12g
Nitrogen gas - 28g
Carbon dioxide - 44g
What is the molar mass of a substance?
The mass of one mole measured in grams
What is one mole?
The Avogadro number of particles (atoms, molecules, ions or electrons) in a substance.
What is the formula for working out the number of moles?
Number of moles = Mass (g)/Mr
What is Avogadro’s law?
One mole of any gas always occupies 24dm3 (24,000cm3) at room temperature and pressure (RTP: 25 degrees and 1 atmosphere)
What is the molar volume?
24dm3
What other two equations can you draw from Avogadro’s law?
Volume (dm3) = moles of gas x 24
Volume (dm3) = mass of gas/ Mr of gas x 24
What does one mole always contains no matter what the element?
6 x 10 23 atoms
What is concentration?
The “amount of stuff” per unit volume.
What are the units for concentration?
1 mole per dm3 (1 mole/dm3)
What is the equation for working out concentration?
Concentration = Number of moles/Volume
What equation can you use to change mol/dm3 - g/dm3?
Mass (g) = Number of moles x Mr
What can a chemical reaction be described as?
The process of going from reactants to products in a reaction
What can you write to show these chemical reactions?
Word and symbol equations
What do state symbols show? and list the four main ones
They show the physical state the reactants and products are in
(s) Solid
(l) Liquid
(g) Gas
(aq) Aqueous - dissolved in water
What do symbol equations always need to be?
Balanced - there must be the same number of atoms on both side
What method should you take for balancing equations?
Balance just one atom at a time - and do it in pencil
What is the empirical formula?
The simplest formula that tells you the ratio of different elements in a compound
What are the simple steps for working out the empirical formula?
1) List all the elements in the compound
2) Underneath write the given masses/percentages
3) Divide each mass by their Ar
4) Turn them into a simple ratio by either x/÷
5) Put ratio into simplest form
What is the molecular formula?
Tells you the actual number of atoms of each element in a single molecule
What is the method for working out the molecular formula?
1) Find the mass of the empirical formula
2) Divide the relative molecular mass by the total mass from 1)
3) Use the answer in the 2) to times it by the empirical formula
What are the three steps for working out masses in reactions?
1) Write out a balanced equation
2) For the two bits you want, work out relative formula (Mr) and multiply them by balancing numbers in the equation
3) Apply the rule: divide to get one, multiply to get all
What is a percentage yield?
The amount of product obtained in a chemical reaction - the more reactant you start off with the higher the yield
How is the theoretical yield of a reaction calculated?
From the balanced equation when calculating masses in reactions
What is a percentage yield always between?
0-100%
What does a 100% yield mean?
You got all the products you expected to get
What does a 0% yield mean?
No reactants were converted into product - no product was made
What two things can salts be?
anhydrous or hydrated
What do all salts consist of?
A lattice of positive and negative ions - some have eater molecules incorporated in the lattice too
What is the water in a lattice called?
Water of crystallisation
A solid salt containing water of crystallisation is….
Hydrated
If a solid doesn’t contain any water of crystallisation it is…
Anhydrous
How can you calculate how much water crystallisation a salt contains?
Use the formula provided in the question - one mole of a hydrated salt always has a particular number of moles of water crystallisation (always a whole number)
What happens to many hydrated salts when they are heated?
They lose their water of crystallisation and become anhydrous
What do you need to know to work out the formula of how much water of crystallisation a salt contains?
Mass of the salt when hydrated and anhydrous
What are the five steps for working out how much water crystallisation a salt contains?
1) Work out the mass of the hydrated and anhydrous salts
2) Find the number of moles of water lost
3) Find the number of moles of the anhydrous salt
4) Work out the ratio of moles of anhydrous salt : moles of water
5) Either round off number or add back into equation
How do you work out percentage yield?
Percentage yield = actual yield (grams)/theoretical yield (grams)
What happens during a chemical reaction?
Old bonds are broken and new bonds are formed
Volume = ?
What must the units of volume be?
Volume = moles * 24
Must be in dm^3
Rules about gases and volume?
1 moles of any gas occupies a volume of 24dm^3
How to convert from cm^3 to dm^3
1000 cm^3 = 1 dm^3
Moles = ? (in solution)
Units?
Moles = concentration * volume
Moles = mol
Concentration = mol / dm^3
Volume = dm^3
Mnemonic for redox reactions?
What is oxidation?
What is reduction?
OIL RIG (electrons)
loss of electrons, gain of oxygen
gain of electrons, loss of oxygen
Half ionic equation that occurs at:
Anode and cathode?
Anode:
Cu(s) - 2e- –> Cu2+(aq)
Cathode:
Cu2+(aq) + 2e- –> Cu(s)
Copper purification:
What happens to the mass of the electrodes?
The cathode gains mass and the anode dissolves into the electrolyte leaving behind anode sludge. The anode loses mass.
Copper purification:
Use of the anode sludge?
It may contain valuable metals such as gold, platinum… These are extracted from the anode sludge and sold.
Name the electrolyte used in copper purification
Commonly: Copper(II) Sulphate
