Formulae, Equations and Amount of Substance

Concentration and Gas Volumes

Pearson Edexcel International A Level Chemistry

Concentration

concentration (mol dm⁻³) = amount (mol) / volume (dm³) c = n / V n = c × V / 1000 with V in cm³

Write the unit of a concentration as mol dm⁻³. Never write g or dm³ mol⁻¹ as the unit of a concentration in moles.
A concentration in g dm⁻³ is a mass per volume: mass (g) / volume (dm³), or concentration in mol dm⁻³ × M.

Molar volume of a gas

amount of gas (mol) = volume of gas / molar volume n = V / 24 with V in dm³ n = V / 24 000 with V in cm³

At room temperature and pressure one mole of any gas occupies 24 dm³, which is 24 000 cm³.
Gases at the same temperature and pressure react in the volume ratio of the coefficients in the balanced equation.
Where a pressure and a temperature are given, use pV = nRT, not the molar volume.

The ideal gas equation

pressure × volume = amount × gas constant × temperature pV = nRT

Quantity	Symbol	Unit
pressure	p	Pa
volume	V	m³
amount of gas	n	mol
gas constant	R	8.31 J K⁻¹ mol⁻¹
temperature	T	K
concentration	c	mol dm⁻³

Unit conversions

Quantity	Change	Do this
volume	cm³ to m³	÷ 1000000
volume	dm³ to m³	÷ 1000
pressure	kPa to Pa	× 1000
temperature	°C to K	+ 273

A volume that comes out in m³ is multiplied by 1000 for dm³.
Rearranged: n = pV/RT for an amount; V = nRT/p for a volume.

Method: a gas calculation

Convert the pressure, the volume and the temperature into Pa, m³ and K.
Write the rearranged form in symbols.
Substitute every value, with R as given, on one line.
Use the amount through the balanced equation, or put the volume into the unit asked for.

Molar mass of a gas or volatile liquid

Amount of gas from its pressure, volume and temperature: n = pV / RT.
Molar mass from the mass of gas: M = m / n, in g mol⁻¹.

Measuring the molar volume of a gas

Gas from the boiling tube passes along the delivery tube and displaces water from the inverted measuring cylinder.

Core practical 1: Measurement of the molar volume of a gas

Put the acid in excess in a boiling tube, with a sealed connection between reaction vessel and delivery tube through the bung and the delivery tube above the level of the liquid in the boiling tube.
Fill a measuring cylinder with water and invert it in a trough, with the water level in trough over the bottom of the measuring cylinder and the end of delivery tube below or in the measuring cylinder.
Weigh the weighing bottle containing the calcium carbonate.
Remove the bung, tip the solid into the acid and quickly replace the bung.
When the reaction has finished, read the volume of gas in the measuring cylinder.
Reweigh the weighing bottle, to take account of any calcium carbonate left in the weighing bottle.
Repeat with increasing masses, plot volume of gas against mass, and read the volume for a chosen mass from the line.
Molar volume = volume of gas / amount of gas, in dm³ mol⁻¹.

With ethanoic acid less gas would escape before the boiling tube was sealed than with hydrochloric acid because ethanoic acid is a weak acid.
Improvement: the solid in a small container inside a sealed flask, with the gas collected in a gas syringe, because there are more divisions on the gas syringe.
Magnesium ribbon held at the top of an inverted burette reacts only once the acid reaches it, as it is at first not in contact with hydrochloric acid. Never write a small surface area of magnesium as the reason the reaction is delayed.
A large mass of magnesium is unsuitable because the volume of hydrogen produced would be greater than burette volume. Never write hydrogen produced too quickly for the burette.

A molar volume below 24 dm³ mol⁻¹

The volume collected is too small because gas was lost or never reached the cylinder: some gas escapes before the bung is attached; some carbon dioxide dissolved in the water; with calcium, some of the calcium had already formed calcium oxide. Never write measurement errors as the reason for a low molar volume or suck-back as the reason for a low molar volume: neither says where the missing gas went.

These notes are part of an Excel with Osman subscription and are not available to print.

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