Formulae, Equations and Amounts of Substance

Question 1

What is a mole?

Answer 1

The mole is the unit for amount of a substance. It is an amount of a substance which contains 6.02 x 10^23 particles of that substance.

Question 2

What is the Avogadro’s constant?

Answer 2

6.02 x 10^23 - it represents the number of particles which make up a mole of a substance. So Avogadro’s constant = number of particles / number of moles.

Question 3

What is the molar mass of a substance?

Answer 3

The molar mass is the mass per mole of the substance in gmol-1. It is the same as relative molecular/formula mass.

Question 4

What equation is used to find the number of moles using the RFM?

Answer 4

Number of moles = mass / molar mass
n = m/Mr

Question 5

What is the equation for finding concentration in moldm-3?

Answer 5

Concentration = moles / volume (dm3)

Question 6

How do you convert concentration from gdm-3 to moldm-3?

Answer 6

gdm-3 / RFM = moldm-3

Question 7

What is the empirical formula of a substance?

Answer 7

The empirical formula is the smallest whole number ratio of atoms of each element in a compound.

Question 8

What is the molecular formula of a substance?

Answer 8

The molecular formula gives the actual number of atoms of each type of element in a molecule. (Only relevant for simple covalent molecules - no other types of structure).

Question 9

How do you calculate empirical formulae from experimental results?

Answer 9

Divide the masses of each substance used/produced by the RFM of that substance. Divide the number of moles of each substance by the smallest number of moles to get the mole ratio. Use this to get the empirical formula.

Question 10

How do you work out empirical formula from percentage composition of a sample?

Answer 10

Do the same as for experimental results, just imagine you have 100g of the substance (so divide by RFM to find moles, divide by the smallest number of moles to find ratio, find empirical formula).

Question 11

How do you work out the percentage composition of an element in a compound?

Answer 11

Percentage composition of element X = total (mass of X in compound / total mass of compound )x100.

Question 12

How do you write an ionic equation?

Answer 12

1. Write out the balanced chemical equation with state symbols.
2. Split ionic compounds into the ions they contain
3. Cross out any spectator ions
4. Rewrite the final ionic equation and include state symbols

Question 13

What are the state symbols?

Answer 13

They show the state a substance is in:
- (s) - solid
- (g) - gad
- (l) - pure liquid
- (aq) - aqueous solution

Question 14

What is the molar gas volume?

Answer 14

The molar gas volume is the volume of space that one mole of a gas occupies at a certain temperature and pressure.

Question 15

What is the molar volume of a gas at r.t.p and s.t.p?

Answer 15

Room temperature and pressure - 24 dm3
Standard temperature and pressure - 22.4 dm3

Question 16

How do you work out the number of moles in a certain volume of gas?

Answer 16

Number of moles = volume in dm3 / molar gas volume (usually 24)

Question 17

What is a displacement reaction?

Answer 17

In a displacement reaction, a more reactive element reacts to take the place of a less reactive element in a compound. This often results on a colour change.

Question 18

What is the ionic equation for the reaction between chlorine and potassium bromide?

Answer 18

Cl2 (aq) + 2KBr (aq) —> 2KCl (aq) + Br2 (aq)

Question 19

What is the general ionic equation for a neutralisation reaction?

Answer 19

H+ (aq) + OH- (aq) —> H2O (l)

Question 20

What is the ionic equation for a neutralisation reaction involving a carbonate?

Answer 20

2H+ (aq) + CO32- (aq) —> H2O (l) + CO2 (g)

Question 21

What is the ionic equation for a neutralisation reaction involving ammonia?

Answer 21

H+ (aq) + NH3 (aq) —> NH4+ (aq)

Question 22

What is a precipitation reaction?

Answer 22

It is when two soluble compounds react to form a solid product (so a precipitate can be seen).

Question 23

What is the ionic equation for the reaction between barium chloride and potassium sulphate?

Answer 23

Ba2+ (aq) + SO42- (aq) —> BaSO4 (s)

Question 24

What is the ideal gas equation?

Answer 24

pV=nRT
pressure x volume = no. of moles x molar gas constant x temperature

Question 25

What are the units of the ideal gas equation?

Answer 25

Pressure - Pa (pascals)
Volume - m3 (meters cubed)
number of moles - mol (moles)
temperature - K (Kelvin)
molar gas constant J K-1 mol-1

Question 26

What is the value of the molar gas constant (R)?

Answer 26

8.3144598 (only need to know 8.31 for A Level)

Question 27

How do you convert from degrees Celsius to kelvin?

Answer 27

K = degrees C + 273.15

Question 28

What are the two main ways of collecting a gas in an experiment?

Answer 28

• connect a delivery tube in a bung to a gas syringe and place in a flask. The volume of gas evolved is the value read off the syringe
• fill a water trough with water and place an upturned measuring cylinder filled with water into the water trough. Place a delivery tube under the measuring cylinder and connect to a bung in the flask. Volume of gas evolved is the difference between the initial and final volumes read off the cylinder.

Question 29

Describe a method for core practical 1 - measuring the molar volume of a gas.

Answer 29

1) use a graduated pipette (low uncertainty so high accuracy) to measure out a known volume of hydrochloric acid into a conical flask either connected to a gas syringe or an upturned measuring cylinder in a water bath.
2) use weighing by difference (weighing boat and scales) to measure a known mass of sodium carbonate
3) add the sodium carbonate to the conical flask and allow the reaction to run to completion
4) record the volume of carbon dioxide collected in the gas syringe/measuring cylinder
5) graph the results and draw a line of best fit
6) read off the graph and use the balanced equation to work out the molar volume

Question 30

What is the purpose of a titration?

Answer 30

Titrations are used to find out the concentrations of acid or alkali solutions.

Question 31

What is a standard solution?

Answer 31

A standard solution is a solution of known concentration.

Question 32

How do you make a standard solution?

Answer 32

1) use weighing by difference to measure an accurate mass of solute
2) add the solute to a beaker and dissolve it in a small amount of distilled/deionised water (predissolving), stir with a glass rod
3) pour the solution into a volumetric flask using a funnel
4) rinse the funnel, beaker and glass rod with distilled/deionised water and pour the rinsings into the volumetric flask
5) fill the volumetric flask with distilled/deionised water up to the mark (bottom of the meniscus should touch the line - if overshot, you must start again)
6) stopper the volumetric flask and invert it multiple times to ensure it is fully mixed and that the concentration of solute is equal throughout

Question 33

What steps can be taken to ensure accuracy in a titration?

Answer 33

• place the conical flask on a white tile so the colour change can be seen more easily
• measure the volumes at eye level from the bottom of the meniscus
• add the solution in the burette dropwise near the end point, to avoid overshooting it
• swirl the flask throughout, so that the acid and alkali fully mix (avoids overshooting end point)
• rinse the burette with your acid before hand so that any impurities are removed (do not use water as this will dilute the solution slightly)
• remove the funnel from the top of the burette after pouring in your solution so that drops do not drip into the burette and affect the volumes
• only use concordant results when calculating mean titre

Question 34

What are concordant results?

Answer 34

Concordant results are titre volumes which are within a certain amount of each other (uncertainty x 2), results should usually be within 0.1cm3 of each other, but this value depends on the burette used.

Question 35

What is the colour change of methyl orange?

Answer 35

Acid - red
Neutral - orange (it is orange at the end point)
Alkali - yellow

Question 36

What is the colour change of phenolphthalein?

Answer 36

Acid - colourless
Neutral - light pink
Alkali - pink

Question 37

Why is it important that not too much indicator is added?

Answer 37

Indicators are weak acids, so adding too much will affect the titre volumes and therefore the calculated concentration.

Question 38

What is the equation needed for titration calculations?

Answer 38

n = c x v
no. of moles = concentration (mol dm-3) x volume (dm3)

Question 39

How do you do a simple titration calculation?

Answer 39

Work out the no. of moles of the substance of which you know both the concentration and volume. Use the balanced chemical equation to determine the moles of the substance of unknown concentration. Use the moles and volume to work out the concentration of the substance in the conical flask.

Question 40

How do you do back titration calculations?

Answer 40

Back titration calculations are slightly more difficult than normal ones, but they are basically the reverse of regular titration calculations. First, you work out the moles of the substance you know the concentration and volume of. Then, you use the mole ratio from the balanced equation to work out the moles of the other substance in the aliquot (sample). Scale up to find the total moles of the solution then use n/v=c to calculate the concentration.

Question 41

What is uncertainty?

Answer 41

Uncertainty is the maximum amount of error a result might have due to the limited sensitivity of the equipment. This is half the smallest increment the equipment can measure, in either direction.

Question 42

How do you calculate percentage uncertainty?

Answer 42

Percentage uncertainty = (number of readings x total uncertainty / value measured) x 100

Question 43

How can percentage uncertainty be reduced?

Answer 43

• use more precise equipment
• use a larger volume/mass of the substance you are measuring wherever possible

Question 44

What are systematic errors?

Answer 44

Systematic errors are errors which are the same every time you repeat the experiment. They are often caused by miscalibrated or inaccurately made equipment.

Question 45

What are random errors?

Answer 45

Random errors are errors that only occur once - they cause repeats of experiments to vary from each other. An example is misreading the value on a burette.

Question 46

Why are experiments repeated multiple times?

Answer 46

Repeating experiments improves the reliability of the results and reduces the effect of random errors.

Question 47

How can the total uncertainty in a result be calculated?

Answer 47

• find the percentage uncertainty for each piece of equipment used
• add the percentage uncertainties together to get the percentage uncertainty in the final result
• multiply this percentage by the result to get the actual total uncertainty.

Question 48

What is the equation for percentage yield?

Answer 48

Percentage yield = (actual yield/theoretical yield)x100

Question 49

What is the equation for atom economy?

Answer 49

% Atom economy = (Mr of desired product / total Mr of all products)x100

Question 50

Why do almost all reactions have atom economies below 100%?

Answer 50

Addition reactions are the only reactions with an atom economy of 100%. All other reactions have a lower atom economy than this for many reasons, for example the reaction may not be run to completion, the reaction may be reversible, some of the product may be lost (e.g. gases escaping), unwanted side reactions may occur, practical losses (e.g. some reactants are not transferred into the reaction vessel).