C4 Chemical Calculations (Chemistry AQA Seperate)
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113 Terms
What is the relative formula mass (Mr)?
The sum of the relative atomic masses (Ar) of all the atoms in a formula.
What is the relative atomic mass (Ar)?
The average mass of an atom compared with 1/12 of the mass of a carbon-12 atom.
How do you calculate Mr?
Add together the Ar values of all atoms in the formula.
What is the Mr of H₂O?
18.
What is the Mr of CO₂?
44.
What is the Mr of NaCl?
58.5.
What is the Mr of MgO?
40.3.
What is the Mr of CaCO₃?
100.
What is the Mr of H₂SO₄?
98.
What is the Mr of CuSO₄?
159.5.
What is a mole?
The amount of substance containing 6.02 × 10²³ particles.
What is Avogadro's constant?
6.02 × 10²³ particles per mole.
What does one mole of any substance contain?
6.02 × 10²³ particles.
What is the equation linking moles, mass and Mr?
Moles = Mass ÷ Mr.
How do you calculate mass from moles?
Mass = Moles × Mr.
How do you calculate Mr from mass and moles?
Mr = Mass ÷ Moles.
What is the unit of moles?
mol.
Why is the mole used in chemistry?
It allows chemists to count particles by weighing substances.
What is the balanced equation for hydrogen reacting with oxygen?
2H₂ + O₂ → 2H₂O.
Why must chemical equations be balanced?
To obey the law of conservation of mass.
What does a coefficient show in a chemical equation?
The mole ratio of substances.
What is the mole ratio in 2H₂ + O₂ → 2H₂O?
2 : 1 : 2.
How are mole ratios used?
To calculate the amount of one substance from another.
What is the equation for percentage by mass?
Percentage by mass = (Mass of element ÷ Mr of compound) × 100.
Why is percentage by mass useful?
It shows how much of a compound is made from each element.
What is the percentage of oxygen in water?
88.9%.
What is the empirical formula?
The simplest whole-number ratio of atoms in a compound.
How do you find an empirical formula?
Convert masses to moles, divide by the smallest value, then simplify to whole numbers.
What is the molecular formula?
The actual number of atoms of each element in a molecule.
How do you calculate the molecular formula?
Divide the molecular mass by the empirical formula mass, then multiply the empirical formula by that number.
What is the law of conservation of mass?
Mass is neither created nor destroyed in a chemical reaction.
Why might mass appear to decrease in an open system?
A gas escapes.
Why should reactions be carried out in a closed system when measuring mass?
To prevent gases escaping.
What is atom economy?
A measure of how much of the reactants end up in the desired product.
What is the equation for atom economy?
Atom economy = (Mr of desired product ÷ Total Mr of reactants) × 100.
Why is high atom economy important?
It reduces waste and improves sustainability.
What is a reversible reaction?
A reaction that can proceed in both directions.
What symbol shows a reversible reaction?
⇌
What is a reversible reaction at equilibrium?
The forward and reverse reactions occur at the same rate.
When is equilibrium reached?
When the forward and reverse reactions occur at equal rates.
Do the reactions stop at equilibrium?
No, they continue at equal rates.
What is a closed system?
A system where no substances enter or leave.
Why is a closed system needed for equilibrium?
So reactants and products cannot escape.
How does increasing concentration affect equilibrium?
It shifts to reduce the added substance.
How does decreasing concentration affect equilibrium?
It shifts to replace the removed substance.
How does increasing pressure affect equilibrium?
It favours the side with fewer gas molecules.
When does pressure have no effect on equilibrium?
When there are equal numbers of gas molecules on both sides.
How does increasing temperature affect an exothermic equilibrium?
It shifts in the endothermic direction.
How does decreasing temperature affect an exothermic equilibrium?
It shifts in the exothermic direction.
What is an exothermic reaction?
A reaction that transfers energy to the surroundings.
What is an endothermic reaction?
A reaction that takes in energy from the surroundings.
Does a catalyst change the position of equilibrium?
No.
What does a catalyst do at equilibrium?
It speeds up both the forward and reverse reactions equally.
What is the Haber process used to make?
Ammonia.
What are the reactants in the Haber process?
Nitrogen and hydrogen.
What is the balanced equation for the Haber process?
N₂ + 3H₂ ⇌ 2NH₃.
What is the source of nitrogen in the Haber process?
The air.
What is the source of hydrogen in the Haber process?
Natural gas (methane) and steam.
What catalyst is used in the Haber process?
Iron.
What temperature is used in the Haber process?
Around 450°C.
What pressure is used in the Haber process?
Around 200 atmospheres.
Why is a high pressure used in the Haber process?
It increases the ammonia yield.
Why isn't an even higher pressure used?
It is expensive and unsafe.
Why is a moderate temperature used in the Haber process?
It balances a good yield with a fast reaction rate.
Is the Haber process exothermic or endothermic?
Exothermic.
Why does increasing temperature reduce ammonia yield?
The equilibrium shifts to the left.
Why is ammonia removed as it forms?
To shift equilibrium towards more ammonia.
What happens to unreacted nitrogen and hydrogen?
They are recycled.
What is ammonia used for?
Fertilisers.
What is the Contact process used to make?
Sulfuric acid.
What gas is oxidised in the Contact process?
Sulfur dioxide.
What is the balanced equation for the Contact process equilibrium?
2SO₂ + O₂ ⇌ 2SO₃.
What catalyst is used in the Contact process?
Vanadium(V) oxide.
What temperature is used in the Contact process?
Around 450°C.
Why is a moderate temperature used in the Contact process?
To balance yield and reaction rate.
Why is only a moderate pressure used in the Contact process?
The pressure effect is small compared with the cost.
Is the Contact process exothermic or endothermic?
Exothermic.
What is the formula for concentration?
Concentration = Mass ÷ Volume.
What are the units for concentration?
g/dm³.
What is the formula for concentration in mol/dm³?
Concentration = Moles ÷ Volume.
What volume unit must be used in concentration calculations?
dm³.
How do you convert cm³ to dm³?
Divide by 1000.
How many cm³ are in 1 dm³?
1000.
What is the equation linking moles, concentration and volume?
Moles = Concentration × Volume (dm³).
How do you calculate concentration?
Concentration = Moles ÷ Volume.
How do you calculate volume?
Volume = Moles ÷ Concentration.
What is the formula for gas volume at room temperature and pressure?
Gas volume = Moles × 24 dm³.
What volume does one mole of any gas occupy at RTP?
24 dm³.
What volume does 0.5 moles of gas occupy at RTP?
12 dm³.
What volume does 2 moles of gas occupy at RTP?
48 dm³.
What is a limiting reactant?
The reactant that is completely used up first.
Why does a limiting reactant matter?
It determines the maximum amount of product formed.
What is excess reactant?
A reactant left over after the reaction finishes.
What is theoretical yield?
The maximum amount of product that could be made.
What is percentage yield?
(Actual yield ÷ Theoretical yield) × 100.
Why is percentage yield often below 100%?
Reactions may be reversible, incomplete or products may be lost.
Why might products be lost during preparation?
During filtration, transfer or purification.
Why do reversible reactions reduce yield?
Not all reactants are converted into products.
What does a balanced symbol equation tell you?
The ratio of moles reacting.
How do you solve mole calculation questions?
Convert to moles, use the mole ratio, then convert back if needed.