Chapter 3: Amounts of Substances

A mole of substance is the amount of that substance that contains the same number of stated elementary units as there are atoms in carbon-12 (ie amount of substance that contains 6.02x10^23 particles, just like in carbon-12)

6.02 x 10^23 mol^-1, the number of particles in each mole of carbon-12.

Molar mass gives the mass in grams in each mole of the substance (mass per mole of substance)

gmol-1

Amount (mol) = mass (g) / molar mass (gmol-1) ie n = m/mr

Molecular formula is the number of atoms of each element in a molecule.

2 or more atoms held together by covalent bonds.

The simplest whole-number ratio of atoms of each element in a compound.

Relative molecular mass Mr compares the mass of a molecule with the mass of an atom of carbon-12.

Relative formula mass compares the mass of a formula unit with the mass of an atom of carbon-12.

Water molecules that are bonded into a crystalline structure of a compound.

• Weigh an empty crucible. • Add the hydrated salt into the crucible. Weigh the crucible and the hydrated salt. • Using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod. Heat the crucible and its contents gently for about 1 minute. Then heat it strongly for a further 3 minutes. • Leave the crucible to cool. Weight the crucible and anhydrous salt. Using your results, calculate the formula of the hydrated salt.

• All of the water has been lost: some water could still be left inside. To combat this, heat until constant mass (reheat repeatedly until the mass of the residue doesn’t change) • No further decomposition (Many salts decompose further when heated, and there may be a colour change, but this can be difficult to judge).

1cm3 = 1ml 1dm3 = 1L 1m3 = 1000L

1moldm-3 solution contains 1mol of solute dissolved in each 1dm3 of solution. n (amount in mol) = c (concentration in moldm-3) x v (volume in dm3) n = c x v

Standard solution is a solution of known concentration.

Dissolve an exact mass of the solute in a solvent and make up the solution to an exact volume.

moldm-3 x Mr = gdm-3 (mass concentration should always be bigger)

The molar gas volume is the volume per mole of gas molecules at a stated temperature and pressure. At the same temperature and pressure, equal volumes of different gases contain the same number of molecules.

RTP = room temperature and pressure At RTP, molar gas volume = 24.0dm3mol-1

amount (mol) = volume / molar gas volume At RTP: n = V / 24

When experiments aren’t at RTP, you use the ideal gas equation to work out the volume of the gas etc.

• In random motion • Elastic collisions (both momentum and KE are preserved i.e. no dissipative forces like friction, heat or sound) • Negligible size • No intermolecular forces (so they don’t attract/repel each other, so behaviour of molecules aren’t affected and therefore can be in random motion)

pV = nRT p pressure in Pa V volume in m3 n amount of gas molecules in mol R ideal gas constant in Jmol-1K-1 T temperature in K