Chemistry - 1 Quantative Chemistry


Moles and Formulas

Mass (m) = number of moles (n) x molar mass (M)

Number of moles (n) = mass (m) / molar mass (M)

Avogadro's constant is 6.02 x 1023 and is the number of atoms in one mole of any element.

Number of particles (N) = number of moles (n) x Avogadro's constant (L)

Empirical formula gives the ratio of the atoms of different elements in a compound (molecular formula represented as simplest ratio)

1. g or % of elements / their Ar = moles
2. Moles / smallest number of moles = simplest ratio

Molecular formula shows actual number of atoms of each element present in molecule.

1. Number given / total Ar = ratio
2. Multiply empirical formula by ratio

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Reactions and equations

2CO shows there is 2 moles of CO (2 is the coefficient and gives info on the molar ratio)  

State symbols - l, g, s, aq  

Ionic equations  are used because ionic compounds are completely dissociated in solution.

Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) à AgCl(s) + Na+(aq) + NO3-(aq)   

Na+(aq) and NO3-(aq) are spectator ions (do not take part), so we make the ionic equation which is…

Ag+(aq) + Cl-(aq) --> AgCl(s)

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Density (ρ) = mass (m) / volume (v)

Concentration (mol dm-3) = number of moles (mol) / volume (dm3)

Divide volume by 1000 if in cm3

Standard solutions are solutions of known concentration. They are used to find concentrations of other solutions which can be done by volumetric analysis such as titrations.

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Calculations from equations

1. Write down the correct formulas for all the reactants and products.

2. Balance the equation to obtain the correct stoichiometry of the reaction.

3. If the amounts of all reactants are known, work out which are in excess and which is the limiting reactant. Knowing the limiting reactant means the maximum yield of of any of the products can be determined.

4. Work out the number of moles of the substance required.

5. Convert the number of moles into the mass or volume.

6. Express the answer to the correct number of sig.fig. and include appropriate units.

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Limiting reactants and yield

If there is insufficient amount of an reactant to react with another reactant then it is the limiting reactant and the other reactant is in excess.

These can be used to work out the theoretical yield.

Often experimental yield < theoretical yield, because...

-reaction is incomplete
-side reactions with unwanted substances produced
-complete separation of product from reaction mixture is impossible
-product lost during transfer of chemicals during preparation

Efficiency of procedure given by percentage yield.

% yield = (experimental yield / theoretical yield) x 100

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Kinetic theory - all matters consist of particles in motion and as temperature increases, the movement of these particles increases (once particles have sufficient energy to overcome interparticle forces, change of state occurs)

Solid - particles closely packed in fixed positions, interparticle forces restrict movement to vibration, fixed shape

Liquid - particles are relatively close together, interparticle forces sufficiently weak to allow particles to change places but movement constrained to fixed volume, can change shape not volume

Gas - interparticle forces are negligible (zero for ideal gas), particles move freely occupying all space available, no fixed shape or volume

T (K) = T (°C) + 273

Note: solid to gas = sublimation, gas to solid = reverse sublimation

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One mole of any gas at STP occupies 22.4 dm3. At RTP it is 24 dm3. These are molar volumes. For a gas:

Number of moles (n) = volume (V) / molar volume (Vmol)

At constant temperature - P ∝ 1/V (or PV = constant)
At constant volume - P ∝ T (or P/T = constant)
At constant pressure - V ∝ T (or V/T = constant)

These variables are all related by the Ideal Gas Equation:

PV = nRT                       (this is used if you are given or trying to find moles)

An ideal gas exactly obeys these laws. Real gases have some attractive forces between particles and these particles do occupy some space so they do not exactly obey. If they did they would never condense into liquids. Gases behave most 'ideally' at high temperatures and low pressures.

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When mass of a particular gas is fixed (nR is constant) and we want to convert pressure, temperature and volume under one set of conditions (1) to another (2) we use the Combined Gas Law:

P1 V1 = P2 V2
   T1         T2

Same units for P and V on each side and T is absolute temperature in K.

Effect of temperature on gas volume:

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