The Mole, Avogadro & The Ideal Gas Equation (AQA A Level Chemistry): Flashcards
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Define the mole in terms of carbon-12.
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Define the mole in terms of carbon-12.
A Mole is the amount of substance that contains the same number of elementary entities as there are atoms in exactly 12.00 g of carbon-12.\n\nOne mole of any element has a mass in grams equal to its Ar.
True or False?
The Avogadro constant applies only to atoms, not to molecules or ions.
False.
The Avogadro constant (L = 6.02 × 1023 mol-1) applies to any elementary entity: atoms, molecules, ions and electrons.
The Avogadro constant L = .......... mol-1. The number of particles in a sample = moles × .......... .
The Avogadro constant L = 6.02 × 1023 mol-1. The number of particles in a sample = moles × L.
How many atoms are present in 1 mol of H2?
1 mol of H2 contains 6.02 × 1023 molecules.
Since each molecule contains 2 H atoms, the total number of atoms = 2 × 6.02 × 1023 = 1.204 × 1024 atoms.
One mole of a compound has a mass equal to its relative .......... mass (Mr) expressed in .......... .
For example, 1 mol of H2O has a mass of .......... g.
One mole of a compound has a mass equal to its relative formula mass (Mr) expressed in grams.
For example, 1 mol of H2O has a mass of 18.0 g.
True or False?
1 mol of NaCl contains 6.02 × 1023 atoms in total.
False.
1 mol of NaCl contains 6.02 × 1023 formula units, each with one Na and one Cl atom. The total number of atoms is 2 × 6.02 × 1023 = 1.204 × 1024.
What does the equation moles = mass / molar mass allow you to calculate?
The number of moles of a substance from its mass in grams and its molar mass in g mol-1.
Rearranging also gives mass = moles × molar mass, used to find the mass of a product or reactant.
True or False?
1 mol of CaF2 contains 1 mol of Ca2+ ions and 1 mol of F- ions.
False.
1 mol of CaF2 contains 1 mol of Ca2+ ions and 2 mol of F- ions, because the formula unit CaF2 has two fluoride ions.
Write the .......... equation. Calculate moles of the known substance. Use the .......... ratio to find moles of the unknown. Multiply moles by .......... mass to find mass.
Write the balanced equation. Calculate moles of the known substance. Use the molar ratio to find moles of the unknown. Multiply moles by molar mass to find mass.
Calculate the maximum mass of MgO produced when 7.5 g of Mg burns in oxygen.
2Mg (s) + O2 (g) → 2MgO (s) (Ar: Mg = 24.3, O = 16.0)
mol(Mg) = 7.5 / 24.3 = 0.309 mol
Ratio Mg : MgO = 1 : 1, so mol(MgO) = 0.309 mol
Mass = 0.309 × 40.3 = 12.4 g
In a combustion experiment, 50 cm3 of propane reacts with 250 cm3 of oxygen, producing 150 cm3 of CO2.
The molar ratio of propane : O2 : CO2 is .......... : .......... : .......... , suggesting the balanced equation is C3H8 + .......... O2 → .......... CO2 + 4H2O.
The molar ratio of propane : O2 : CO2 is 1 : 5 : 3, suggesting the balanced equation is C3H8 + 5 O2 → 3 CO2 + 4H2O.
True or False?
The stoichiometry of a reaction can be deduced from the volumes of gases that react and are produced.
True.
Because equal volumes of gases contain equal numbers of molecules (Avogadro's hypothesis), volume ratios equal mole ratios for gases.
Stoichiometry
The molar ratio of reactants and products in a balanced chemical equation, showing the exact proportions in which substances react and are formed.
Define concentration of a solution.
A Concentration is the amount of solute (in mol) dissolved per dm3 of solution.\n\nconcentration (mol dm-3) = moles of solute / volume (dm3)
True or False?
At room temperature and pressure, one mole of any gas occupies 24.0 dm3.
True.
AQA defines room temperature and pressure (RTP) as 25 °C (298 K) and 100 kPa. This molar gas volume applies to all ideal gases under these conditions.
Volume of gas (dm3) = amount (mol) × ..........
Amount of gas (mol) = volume (dm3) ÷ ..........
Volume of gas (dm3) = amount (mol) × 24.0
Amount of gas (mol) = volume (dm3) ÷ 24.0
Calculate the volume of 1.0 mol dm-3 HCl required to react completely with 2.5 g of CaCO3.
CaCO3 + 2HCl → CaCl2 + H2O + CO2 (Mr CaCO3 = 100)
mol(CaCO3) = 2.5 / 100 = 0.025 mol
mol(HCl) = 0.025 × 2 = 0.050 mol
Volume = 0.050 / 1.0 = 0.050 dm3
When working with concentrations in mol dm-3, volumes in cm3 must be converted to dm3 by .......... by .......... .
Masses in grams must be converted to .......... before use in concentration calculations.
When working with concentrations in mol dm-3, volumes in cm3 must be converted to dm3 by dividing by 1000.
Masses in grams must be converted to moles before use in concentration calculations.
True or False?
A concentrated solution always contains more moles of solute than a dilute solution of the same volume.
True.
Concentration is defined as moles per dm3. For the same volume, a higher concentration means more moles of solute are dissolved.
What is an ideal gas?
An Ideal gas is a gas that obeys all assumptions of the kinetic theory: particles move randomly, have negligible volume, exert no intermolecular forces, undergo perfectly elastic collisions, and have average kinetic energy directly proportional to absolute temperature.
Complete the ideal gas equation:
Symbol
Quantity
Unit
P
..........
..........
V
..........
..........
n
..........
..........
R
gas constant
..........
T
..........
..........
Symbol
Quantity
Unit
P
pressure
Pa
V
volume
m3
n
moles of gas
mol
R
gas constant
8.31 J K-1 mol-1
T
temperature
K
True or False?
At very high temperatures and low pressures, real gases deviate most significantly from ideal behaviour.
False.
Real gases deviate most from ideal behaviour at very low temperatures and high pressures, where intermolecular forces become significant and the volume of particles is no longer negligible.
A gas occupies 0.00867 m3 at 220 kPa and 294 K. How many moles of gas are present? (R = 8.31 J K-1 mol-1)
Use n = PV / RT:
n = (220 000 × 0.00867) / (8.31 × 294) = 0.781 mol
When a gas is heated at constant pressure, particles gain kinetic energy and collide .......... with container walls. The volume .......... to maintain constant pressure.
When a gas is heated at constant pressure, particles gain kinetic energy and collide more frequently and with greater force with container walls. The volume increases to maintain constant pressure.
Define the kinetic theory of gases.
A Kinetic theory of gases is a model describing the behaviour of gas particles. Key assumptions: particles move rapidly and randomly, have negligible volume, exert no intermolecular forces, collide elastically, and their average kinetic energy is proportional to absolute temperature.
Why does the measured pressure of a real gas at high pressure fall below the value predicted by the ideal gas equation?
At high pressure, particles are close together and intermolecular attractive forces become significant. These forces pull particles away from the container walls, reducing the force of collisions and therefore lowering the measured pressure.
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