Define atomic mass unit .

The atomic mass unit (u) is approximately the mass of a proton or neutron, equal to 1.66 × 10 -27 kg.

The mole is the SI base unit of amount of substance; it is the amount containing as many particles as there are atoms in 12 g of carbon-12.

Define Avogadro's constant .

Avogadro's constant ( N A ) is the number of atoms of carbon-12 in 12 g of carbon-12, equal to 6.02 × 10 23 mol -1 .

The molar mass of a substance is the mass, in grams, in .......... .

The molar mass of a substance is the mass, in grams, in one mole .

True or False? Two different gases with the same number of moles, at the same temperature and pressure, must have the same mass.

False. Equal moles of different gases contain the same number of particles, but since their molar masses differ, their masses also differ.

State the assumptions of the ideal gas model.

Molecules have negligible volume Collisions between molecules are elastic The gas cannot be liquefied There are no interactions between molecules except during collisions The gas obeys the ideal gas laws at any temperature or pressure

In the ideal gas equation, the temperature T must always be measured in .......... , not degrees Celsius.

In the ideal gas equation, the temperature T must always be measured in kelvin , not degrees Celsius.

True or False? Collisions between the molecules of an ideal gas result in a loss of kinetic energy.

False. Collisions between ideal gas molecules are perfectly elastic , so no kinetic energy is lost in collisions.

When a gas is compressed at constant temperature, its pressure .......... .

When a gas is compressed at constant temperature, its pressure increases .

How is absolute zero estimated using a Charles's law graph?

Volume is plotted against temperature (in °C) at constant pressure, and the straight-line graph is extrapolated backwards to the temperature at which the volume would be zero.

True or False? The straight-line graph of volume against temperature in degrees Celsius for Charles's law passes through the origin.

False. The graph only passes through the origin when temperature is measured in kelvin ; in degrees Celsius, extrapolating back gives a volume of zero at absolute zero (−273 °C), not at 0 °C.

A LevelPhysicsOCRFlashcards5. Newtonian World & AstrophysicsIdeal Gases

Ideal Gases (OCR A Level Physics): Flashcards

Exam code: H556

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Define atomic mass unit.

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Cards in this collection (21)

Define atomic mass unit.
The atomic mass unit (u) is approximately the mass of a proton or neutron, equal to 1.66 × 10^-27 kg.
Define the mole.
The mole is the SI base unit of amount of substance; it is the amount containing as many particles as there are atoms in 12 g of carbon-12.
Define Avogadro's constant.
Avogadro's constant (N_A) is the number of atoms of carbon-12 in 12 g of carbon-12, equal to 6.02 × 10²³ mol^-1.
How can the number of atoms N in a sample be found from the number of moles n?
$N = n \times N_{A}$
The molar mass of a substance is the mass, in grams, in ...........
The molar mass of a substance is the mass, in grams, in one mole.
True or False?
Two different gases with the same number of moles, at the same temperature and pressure, must have the same mass.
False.
Equal moles of different gases contain the same number of particles, but since their molar masses differ, their masses also differ.
Calculate the mass, in kg, of a carbon-12 atom.
$12 u = 12 \times 1.66 \times 10^{- 27} = 1.99 \times 10^{- 26} kg$
Define an ideal gas.
An ideal gas is a gas that obeys the equation of state $p V = n R T$ at all pressures, volumes and temperatures.
State the assumptions of the ideal gas model.
- Molecules have negligible volume
- Collisions between molecules are elastic
- The gas cannot be liquefied
- There are no interactions between molecules except during collisions
- The gas obeys the ideal gas laws at any temperature or pressure
In the ideal gas equation, the temperature T must always be measured in .........., not degrees Celsius.
In the ideal gas equation, the temperature T must always be measured in kelvin, not degrees Celsius.
Write the ideal gas equation in terms of the number of molecules N and the Boltzmann constant k.
$p V = N k T$
Write the ideal gas equation in terms of the number of moles n and the molar gas constant R.
$p V = n R T$
True or False?
Collisions between the molecules of an ideal gas result in a loss of kinetic energy.
False.
Collisions between ideal gas molecules are perfectly elastic, so no kinetic energy is lost in collisions.
How is the ideal gas equation rearranged to find the number of moles n?
$n = \frac{p V}{R T}$
Define Boyle's law.
At constant temperature, the pressure of a fixed mass of ideal gas is inversely proportional to its volume: $P_{1} V_{1} = P_{2} V_{2}$
Define the pressure law.
At constant volume, the pressure of a fixed mass of gas is directly proportional to its thermodynamic temperature: $\frac{P_{1}}{T_{1}} = \frac{P_{2}}{T_{2}}$
Define Charles's law.
At constant pressure, the volume of a fixed mass of gas is directly proportional to its thermodynamic temperature: $\frac{V_{1}}{T_{1}} = \frac{V_{2}}{T_{2}}$
When a gas is compressed at constant temperature, its pressure ...........
When a gas is compressed at constant temperature, its pressure increases.
How is absolute zero estimated using a Charles's law graph?
Volume is plotted against temperature (in °C) at constant pressure, and the straight-line graph is extrapolated backwards to the temperature at which the volume would be zero.
The pressure inside a bicycle tyre is 5.10 × 10⁵ Pa at 279 K. Assuming constant volume, calculate the new pressure when the temperature rises to 299 K.
$P_{2} = \frac{P_{1} T_{2}}{T_{1}} = \frac{5.10 \times 10^{5} \times 299}{279} = 5.47 \times 10^{5} Pa$
True or False?
The straight-line graph of volume against temperature in degrees Celsius for Charles's law passes through the origin.
False.
The graph only passes through the origin when temperature is measured in kelvin; in degrees Celsius, extrapolating back gives a volume of zero at absolute zero (−273 °C), not at 0 °C.

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