Chemistry - 3.1.2: Amount of Substance

Relative atomic mass

The mean mass of an atom of an element, relative to 1/12th of the mean mass of an atom of carbon-12.

Relative molecular mass

The mean mass of a molecule, relative to 1/12th of the mean mass of an atom of carbon-12.

Avogadro's constant

The total number of particles in a mole of substance (6.022 x 10^23)

Mole

The amount of substance that contains 6.022 x 10^23 particles

Moles, mass and Mr formula

moles = mass/Mr

Units for moles

mol

Units for concentration

mol dm-3

Concentration and moles formula

concentration = moles/volume

What is 1 dm3 and 1 cm3 the same as?

1 dm3 = 1 L
1 cm3 = 1 ml

How to find the uncertainty of a measurement?

Halve the value of the smallest division (e.g. if go up in increments of 1 cm, the error would be +-0.5 cm)

How to calculate percentage error?

(total uncertainty/measured value) x 100

Ideal gas equation

For 'n' number of moles:
pressure x volume = moles x gas constant x temperature
P x V = n x R x T

Empirical formula

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

How to calculate empirical formula:

1. Find masses of each element
2. Find moles of each element (moles = mass/Mr)
3. Use moles of each element to find simplest whole number ratio

Molecular formula

The actual number of atoms of each element in a molecule

Spectator ions

Ions that do not participate in a reaction

How to find ionic equation for a reaction?

1. Write out equation in full, separating each ionic compound
2. Eliminate spectator ions (ions on both sides of the equation)
3. Re-write the equation

Titration steps (6)

1. Fill burette with vol of acid of known concentration
2. Measure vol alkali of unknown concentration with a pipette
3. Add alkali to conical flask with few drops of indicator
4. Add acid to alkali with burette till colour of indicator changes
5. Repeat, adding the acid drop-wise when close to end-point of reaction until you get 3 results of the volume of the acid that are the same (within the experimental error)
6. Use equations to work out concentration alkali
(note: alkali could be of known conc instead, while acid is of unknown conc)

Atom economy (definition + formula)

The number of atoms used in a reaction that become the desired product.
Atom economy = (desired product mass/total reactants mass) x 100

Percentage yield (definition + formula)

The amount of a desired product you make in a reaction compared to its maximum theoretical amount.
Percentage yield = (actual yield/theoretical yield) x 100

4 reasons why the yield of a reaction isn't always the maximum theoretical yield

1. Other reactions might've taken place simultaneously
2. The reaction does not go to completion
3. Reactants/products are lost to atmosphere
4. Some reactants/products might be left on equipment (e.g. a container)

Costs of using reactants in excess

1. Waste is produced - may be toxic, damages environment + harms people (environmental)
2. Reactant may be expensive (economic)

2 ways to reduce costs of a low percentage yield?

1. Sell excess metals
2. Recycle excess metals

4 reasons why you might choose a certain reaction over another?

1. Higher percentage yield
2. More valuable products
3. Products that can be recycled
4. Cheaper reactants

How to calculate percentage purity?

(mass pure/total mass impure) x 100

Importance of a high percentage yield?

Get as much product as possible in a reaction / More efficient conversion from reactants to products

Importance of a high percentage atom economy?

Maximise the amount of reactants that become desired products / Minimise the amount of by-products

When carrying out a titration, why should you rinse the burette with the titrant (e.g. NaOH) rather than water?

Using water would dilute the titrant, so it would give a larger titre value.

When carrying out a titration and nearing the end-point of the reaction, why should you rinse the inside of the conical flask with deionised water?

Ensure that all the reactants are in the mixture - some reactants may be stuck to sides of the flask

Concordant titres

Titres that are within 0.1 cm3 of each other

When calculating the Mr of a gas using the ideal gas equation, why might the experimental Mr differ from the actual Mr? (3)

1. Air leaked into the syringe, so the volume of the gas used was too big - if V is too large, Mr is too small
2. Temperature measured is less that temperature inside the syringe (syringe heated faster + oven doesn't heat at constant temperature) - if T is too small, Mr is too small
3. Not all liquid transferred - actual volume lower than that used in calculation, so smaller moles, Mr is too big

Safety precaution when dealing with toxic gases

Carry out experiments in a fume cupboard to avoid toxic vapour

When doing a titration, why should you use a pipette instead of a measuring cylinder to measure the volume?

Measuring cylinder has too large uncertainty / too inaccurate

When doing a titration, when do you stop adding the alkali solution and swirling?

When there is a permanent colour change - if stop too early, not all acid might have reacted.

Why remove funnel when carrying out titration?

Additional drops of solution may enter burette from funnel

Why use a conical flask and not a beaker?

Less chance of spilling when swirling

How to calculate percentage uncertainty?

(uncertainty / measurement) x 100
- if there are 2 different measurements, calculate uncertainty for each then add if overall - if for 1 single measurement but two uncertainties, add total uncertainty over total measurement
- do NOT multiply uncertainty by 2

During a titration, why can you wash the inside of the conical flask with distilled water without it giving an incorrect result?

It does not change the number of moles of the solution in the flask

Give 5 potential improvements when preparing a standard solution

1. Record all masses
2. Weigh by difference OR wash solid from weighing container into the beaker
3. Wash the beaker / stirring rod into the flask after solution is transferred to volumetric flask OR transfer washings to volumetric flask
4. Use a dropper when adding close to graduation mark to ensure bottom of meniscus is on the graduation mark
5. Invert final solution many times OR mix thoroughly the final solution in the volumetric flask

From the moles of the sample and of the bromine water needed to decolourise it, how can you find the number of C=C bonds?

Find the ratio of the sample moles to bromine water moles as 1:x - the value of x is the number of C=C bonds

When carrying out a titration, what would happen if there was a gap between the tap and the tip of the burette that contained air?

Measured volume would be greater as the level in the burette would fall as tap is filled before any liquid is delivered