3.1.2.2 the mole & avogadro’s constant

give the basic volume conversions

1 dm = 10cm → 1dm³ = 1000cm³

1 m = 10 dm → 1m³ = 1000dm³

1m³ = 1000000cm³

what is the mole?

a unit to measure an amount of substance

what is 1 mole?

the amount of substance that contains 6.022×10²³ particles (the same number of particles as there are carbon atoms in 12.0 grams of carbon)

why is the mole a useful quantity?

because one mole of a substance always contains the same number of entities of the substance

give the equation to calculate the number of particles or moles in a substance

number of particles = avogadro constant x number of moles

what can the ‘number of particles’ refer to?

number of:

• atoms

• molecules

• or ions

what is the Avogadro’s constant known as?

L

how can you calculate the number of ions in a substance?

• calculate number of particles using AC x moles

• calculate number of ions present in substance eg in NaCl there are 2 present as there is one Na⁺ + one Cl⁻

• multiply number of particles by number of ions

give the equation to calculate moles

moles (mol) = mass (g)/Mr

if a substance is a liquid how may its mass be determined?

by using its volume + density

give the equation linking density, mass + volume including units

mass (g) = density (gcm⁻³) x volume (cm³)

having a balanced symbol equation gives you what?

the ratio of molecules in the reaction

what can you use the ratio of molecules for?

once you know the moles of one of the species, you can deduce the moles of the other species in the reaction using the ratio

eg using the equation N₂ + 3H₂ → 2NH₃, calculate how many moles of hydrogen + ammonia we would have if there was 5 moles of nitrogen

hydrogen → 15 mol

ammonia → 10 mol

because the ratio is 1 : 3 : 2

why can’t you compare the mass to mass ratio to work out other masses rather than the moles to moles ratio?

because the mass of each molecule will be different

what mass of calcium oxide would be formed if 30.0g of calcium carbonate was decomposed? outline the steps

mass of calcium oxide = 16.8g

when calculating reacting masses, if not stated how many sig figs should you give the answer to?

3 sig figs

in chemical reactions one of the reagents is usually ______ and one is usually in ______

• limiting

• excess (more than necessary)

where do you need to work out which reagent is in excess?

in calculations

the reagent not in excess is known as the what?

the limiting reagent

how do you figure out which reactant is the limiting reagent?

• look at one of the reactants + the mole to mole ratio

• see if the number of mols ‘fits into’ the ratio → if it is in excess or there’s not enough it

eg if the ratio of the reactants (Ca + O₂) is 2 : 1 and there is 10 moles of Ca and 2 moles of O₂ then the reacting reagent is oxygen as there should be 5 mols of it

once you have figured out the limiting reactant how do you work out the number of moles of the product?

using the limiting reagent (NOT the one in excess) → work out the moles using the ratio

5.00g of iron + 5.00g of sulphur are heated together to form iron (II) sulphide. what is the maximum mass of iron (II) sulphide that can be formed? outline the steps

limiting reactant is iron (sulphur is in excess)

→ the mass of iron sulphide is 7.99g