Ideal gases (A-level only) Flashcards – AQA A-level Physics (7408)

What is absolute zero on the Kelvin and Celsius scales?

The zero of the thermodynamic temperature scale: $0\,\mathrm{K}$, equivalent to $-273.15\,^{\circ}\mathrm{C}$.

How are Kelvin and Celsius temperature intervals related in size?

They have the same size.

What equation relates Kelvin temperature $T$ to Celsius temperature $\theta$?

$\frac{T}{\mathrm{K}}=\frac{\theta}{^{\circ}\mathrm{C}}+273.15$

How can absolute zero be estimated from a gas at constant pressure?

Extrapolate the approximately linear volume–Celsius temperature relationship to zero volume.

How can absolute zero be estimated from a gas at constant volume?

Extrapolate the approximately linear pressure–Celsius temperature relationship to zero pressure.

Why does gas extrapolation not imply that an actual gas remains gaseous down to absolute zero?

A real gas can change state before the extrapolated point is reached.

What is the ideal gas equation for an amount $n$ in moles?

$pV=nRT$, where $p$ is pressure, $V$ is volume, $T$ is absolute temperature and $R$ is the molar gas constant.

What is the ideal gas equation for $N$ molecules?

$pV=NkT$, where $p$ is pressure, $V$ is volume, $T$ is absolute temperature and $k$ is the Boltzmann constant.

How is the number of particles $N$ related to the amount $n$ in moles?

$N=nN_A$, where $N_A$ is the Avogadro constant: the number of constituent particles per mole.

What relationship between $R$, $N_A$ and $k$ makes the two forms of the ideal gas equation equivalent?

$R=N_Ak$

What SI units are used for pressure, volume, temperature and amount in the ideal gas equation?

Pressure: pascals; volume: cubic metres; temperature: kelvin; amount: moles.

What is the work done by a gas when its volume changes against a constant pressure $p$?

$W=p\Delta V$, where $\Delta V=V_{\text{final}}-V_{\text{initial}}$.

Why does pressure multiplied by volume change represent work done by a gas?

Pressure is force per unit area; multiplying it by the volume change gives force times the distance moved by the boundary.

What are the signs of the work done by a gas during expansion and compression?

Expansion: positive. Compression: negative.

How does the work done on a gas relate to the work done by the gas?

They have opposite signs.

What unit of work results when pressure is in pascals and volume change is in cubic metres?

Joules.

What is molar mass?

The mass per mole of a substance.

What is molecular mass?

The mass of one molecule.

How is the amount of substance $n$ calculated from sample mass $m$ and molar mass $M$?

$n = \frac{m}{M}$ The mass units used for $m$ and $M$ must be consistent.

A substance has molar mass $M$, and one mole contains $N_A$ molecules. What is the mass of one molecule?

$m_{\text{molecule}} = \frac{M}{N_A}$

A sample contains $n$ moles, and one mole contains $N_A$ molecules. How many molecules does the sample contain?

$N = nN_A$

What units may be used for molar mass and the corresponding molecular mass?

Molar mass: $\mathrm{kg\,mol^{-1}}$ or $\mathrm{g\,mol^{-1}}$. Molecular mass: kilograms or grams per molecule, respectively.