C3-4. Moles And Calculations

Question 1

What is the definition of an acid?

Answer 1

• An acid is a substance which dissociates in solution to release H+ ions.
• Therefore they are also referred to as proton donors.

Question 2

What is the definition of alkali and base?

Answer 2

• Bases are substances which can accept H+ ions from other molecules.
• Alkalis are soluble bases.

Question 3

What is a neutralisation reaction?

Answer 3

This is a reaction between H+ and OH- ions to form water.

Question 4

What determines strong and weak acids?

Answer 4

Strong acids fully dissociate in solution to release H+ ions, whereas weak acids only partially dissociate in a reversible reaction.

Question 5

What are polyprotic acids?

Answer 5

These are acids such as H2SO4 which contain more than one hydrogen atom. One H+ ion is released, and the resulting 1- ion partially dissociates.

Question 6

What is percentage yield defined as?

Answer 6

The amount of product we produce during a reaction as a percentage of the amount we expect to form.

Question 7

What factors affect percentage yield?

Answer 7

• Incomplete reactions
• Some product may remain within the reaction vessel
• Unexpected alternate reactions involving our reactants may have also occurred
• Some product may be lost during purification

Question 8

Define atom economy.

Answer 8

• Atom economy is a measure of how much of the total product formed during a reaction is considered useful.
• Note that we do consider the coefficients of substances involved here.

Question 9

What is the difference between empirical and molecular formulae?

Answer 9

• Molecular formulae provide the actual number of atoms present in a substance.
• Empirical formulae represent the simplest ratio of atoms present in a substance.

Question 10

How can we work out empirical formulae from reaction data?

Answer 10

Generally, you should calculate the number of moles of ‘items’ involved in the formula of the unknown compound:
- C and H for hydrocarbons
- The salt and water of crystallisation for hydrated salts.
This allows you to calculated ratios of atoms in a given compound.

Question 11

What calculations are generally used to process titration data?

Answer 11

• Generally, we need to work towards determining the number of moles present in the reacting solution, and the using that to determine concentration, mass etc. of the unknown substance.
• Remember that we may need to scale the number of moles, e.g. from 25cm3 in a burette to 250cm3 in a flask.

Question 12

What is molar gas volume?

Answer 12

The volume a gas occupies at room temperature and pressure (RTP), generally given as 24dm3.

Question 13

What is the ideal gas equation?

Answer 13

pV=nRT
- i.e. pressure * volume = amount * ideal gas constant * change in temperature
- With units given in Pa, dm3, mol, J mol-1 K-1, and K respectively.

Question 14

What is the ideal gas constant?

Answer 14

8.314 J mol-1 K-1

Question 15

What is a standard solution?

Answer 15

A solution of a known concentration, made by dissolving an exact mass of a solute in a solvent before making up the solution to an exact volume.

Question 16

Why do we complete a rough titration?

Answer 16

To give us a general idea of the endpoint of the titration, so that we can be more accurate in future.

Question 17

How do we record a mean titre, and why do we do this?

Answer 17

• We repeat the titration until two or more results fall within 0.05cm3 of one another, i.e. they are concordant.
• We then calculate an average from these values only.
• We do this to eliminate anomalous results from other attempts.

Question 18

Why do we wash out the beaker and include the washings when we are making up a standard solution?

Answer 18

We do this to ensure that the entire measured amount of the solute we added to our solvent ends up in the solution we are going to make up.

Question 19

Why do we weigh by difference when measuring the mass of the solid used to form a standard solution?

Answer 19

To allow us to calculate the true mass of solid added, accounting for any solid remaining in the weighing boat.

Question 20

How can we ensure that we correctly identify when the endpoint of a titration is reached?

Answer 20

Place the conical flask on top of a white tile to make colour changes clearer. Use a suitable indicator with a noticeable colour change.

Question 21

What is the definition of a hydrated salt?

Answer 21

• If a salt is hydrated, water molecules form part of the crystalline structure.
• We call this water the water of crystallisation, which is lost upon heating to form an anhydrous salt.

Question 22

How do we determine the limiting reagent of a reaction?

Answer 22

Calculate the number of moles of each reactant, and compare this to the ratio of co-efficients in the chemical equation.

Question 23

Why do we need to consider the limiting reagent in a reaction?

Answer 23

One of the reactants may be used up sooner than the other, meaning that this causes the reaction to stop whilst another reactant is left in excess, inhibiting the formation of other products - less may be produced than otherwise expected.

Question 24

If a reaction only has one product, what can we say about its atom economy?

Answer 24

Atom economy must be 100%, as only the desired product is produced, with no by-products.

Question 25

Why is it important to choose a reaction pathway which maximises atom economy?

Answer 25

• It makes industrial processes more efficient; less onwards processing
• It preserves finite raw materials
• It reduces the formation of potentially harmful waste products

Question 26

What is the equation linking mass, molar mass and amount of substance?

Answer 26

m=Mn
- i.e. mass = molar mass * amount
- With values given in units g, g mol-1, and mol respectively.

Question 27

What is the equation linking, amount of substance, moles and concentration?

Answer 27

n=Cv
- i.e. amount = concentration * volume
- With values given in units mol, mol dm-3, and dm3 respectively.

Question 28

What is the formula for the molar gas volume at RTP?

Answer 28

volume = 24n for volume in dm-3
volume = 24000n for volume in cm-3
- Where n is amount in mol

Question 29

What are some major sources of error in moles/composition determination experiments?

Answer 29

• Inaccuracies in measurement of added mass
• Loss of gaseous substances due to flaws in the method
• Inaccuracies in use of measurement equipment

Question 30

What could be considered sources of error when completing titrations?

Answer 30

• Missing the end-point
• Inaccuracies when making up standard solutions
• Airlocks/air bubbles in the burette
• Standard solution not being fully transferred between equipment

Question 31

How could an air bubble in a burette affect the results of a titration?

Answer 31

• The measured volume of standard solution leaving the burette would increase as the air bubble would be included
• So more standard solution would be required to achieve neutralisation
• So the titre increases

Question 32

How would forgetting to include washings from other equipment affect the titre?

Answer 32

• The number of moles of alkali present in the ‘standard’ solution would be less than expected
• So more ‘standard’ solution would be required for neutralisation
• So the titre increases

Question 33

How would forgetting to remove the funnel from the top of the burette affect your titre?

Answer 33

• The difference in volume of standard solution would appear smaller, because solution may drip into the burette during the titration
• So the recorded titre would decrease

Question 34

How might we practically identify an unknown group 2 metal?

Answer 34

• Measure the mass of the metal to be identified.
• We would add dilute hydrochloric acid to a conical flask with a bung, attached to a capillary tube connecting to a gas syringe set to zero.
• The bung should be momentarily removed to add the metal to the hydrochloric acid, before being immediately replaced.
• The volume of acid produced during the reaction can then be recorded; assuming RTP, this enables us to calculate the number of moles of gas produced.
• We can use the standard group 2 metal + acid reaction formula (X+2HCl=XCl2+H2) to determine the moles of metal that reacted.
• This enables us to then determine molar mass.

Question 35

Why must we immediately replace the bung when completing the identifying unknown metals practical?

Answer 35

• To prevent gases produced during the reaction from escaping as much as is possible using this method.
• If gases escape, the number of moles of hydrogen gas produced will be lower, so the measured molar mass of the metal will be larger (the same mass divided by a smaller no. of mole).

Question 35

What sources of error can arise during the identifying unknown metals practical?

Answer 35

• Loss of gaseous products immediately after adding the metal, reducing recorded volume of gas produced
• Changes in atmospheric pressure and/or temperature if RTP is assumed
• Zero error if the gas syringe is improperly reset