Topic 5 - Formulae, Equations & Amounts Of Substances Flashcards
What is the unit for the amount of a substance?
Mole
What is the symbol for the number of moles?
n
What is the Avogadro constant?
- 6.02 x 10²³
- It is the number of particles in a mole
(NOTE: The value is given in the exam!)
What is molar mass?
The mass per mole of something.
What is the definition of molar mass in terms of Mr?
It is the number of particles which weigh the same as the relative molecular mass, Mr.
What are the units for Mr?
g/mol
What is the formula for number of moles?
Moles = Mass / Mr
What are the two units for the concentration of a solution?
- mol/dm³
* g/dm³
What is the equation for the concentration of a solution relative to the number of moles?
Concentration = Moles / Volume
How can you convert from cm³ to dm³?
Divide by 1000.
What is the empirical formula?
A formula with the smallest whole number ratio of atoms of each element.
What is the molecular formula?
A formula with the actual number of atoms of each element.
When a hydrocarbon is burnt in excess oxygen, 4.4g of CO₂ and 1.8g of water are made. What is the empirical formula of the hydrocarbon?
- Moles of CO₂ = 4.4 / (12 + 2 x 16) = 0.10 moles
- Therefore, there are 0.10 moles of C
- Moles of H₂O = 1.8 / (2 x 10 + 16) = 0.10 moles
- Therefore, there are 0.20 moles of H
- Ratio of C:H = 1:2
- So the empirical formula = CH₂
A compound is found to have percentage composition 56.5% potassium, 8.70% carbon and 34.8% oxygen by mass. Calculate its empirical formula.
In 100g of compound: • Moles of K = 56.5 / 39.1 = 1.45 moles • Moles of C = 8.70 / 12.0 = 0.725 moles • Moles of O = 34.8 / 16.0 = 2.18 moles • Ratio of K:C:O = 2:1:3 • So the empirical formula = K₂CO₃
What is the percentage composition of H in CH₄?
(4 x 1.0) / (12.0 + 4 x 1.0) x 100% = 25%
When 4.6g of an alcohol, with molar mass 46g/mol, is burnt in excess oxygen, it produces 8.8g of carbon dioxide and 5.4g of water. Calculate the empirical formula for the alcohol and then its molecular formula.
- Moles of CO₂ = 8.8 / 44 = 0.20 moles
- Therefore, moles of C = 0.20 moles
- Moles of H₂O = 5.4 / 18 = 0.30 moles
- Mass of C = 0.20 x 12.0 = 2.4g
- Mass of H = 0.60 x 1.0 = 0.60g
- Mass of O = 4.6 - 2.4 - 0.60 = 1.6g
- Number of moles of O = 1.6 / 16.0 = 0.10 moles
- Ratio of C:H:O = 2:6:1
- Therefore, empirical formula = C₂H₆O
- Mass of empirical formula = 46.0g
- In this case, the mass of the empirical formula equals the molecular mass, the the two formulae are the same.
- Molecular formula = C₂H₆O
Remember to practise balancing equations.
Pg 58 of revision guide
What do ionic equations show?
Only the resting particles (without spectator ions).
Should charge balance in ionic equations?
Yes
Remember to practise writing ionic equations.
Pg 58 of revision guide
What is reaction stoichiometry?
The ratios of reactants to products in a reaction (i.e. how many moles of product are formed from a certain number of moles of reactants).
What are the 4 state symbols?
- Solid (s)
- Liquid (l)
- Gas (g)
- Aqueous solution (aq)
What happens in a displacement reaction?
A more reactive element replaces another in a compound.
What is produced in a reaction of an acid with a base?
- Salt
* Water
What is formed in a precipitation reaction?
A solid
Which produced in a reaction of an acid with a carbonate?
- Salt
- Water
- Carbon dioxide
What is molar gas volume?
The volume that one mole of a gas occupies at a certain temperature and pressure.
What are the units of molar gas volume?
dm³/mol
What can be said about the volume of any gas at the same temperature and pressure?
It is the same for any gas.
What is the molar gas volume at RTP (20°C and 101.3kPa)?
24 dm³/mol
What is the molar gas volume at STP (0°C and 101.3kPa)?
22.4 dm³/mol
What is the formula for the number of moles of a gas relative to the volume?
Moles = Volume / Molar gas volume