Amount of substance

Question 1

What is amount of substance measured in?

Answer 1

Mole (n)

Question 2

How much is one mole roughly and what can this also be referred to as?

Answer 2

6.02 x 10 23
Avogadro constant
e.g.
1 mole of C contains 6.02 x 10 23 atoms.
1 mole of CH4 contains 6.02 x 10 23 molecules.
1 mole of Na+ ions contains 6.02 x 10 23 ions.
1 mole of electrons contains 6.02 x 10 23 electrons.

Question 3

What is the formula involving Avogadro’s constant?

Answer 3

No. of particles = No.of moles x Avogadro’s constant

Question 4

What is the formula linking moles and Mr and the units?

Answer 4

Mass (g) = Mr x No. of moles/molar mass

Question 5

Define concentration? What are its units?

Answer 5

The concentration of a solution is how many moles are dissolved per 1 dm3 of solution. Units are mol dm-3

Question 6

What is the formula for concentration?

Answer 6

No. of moles= Conc. (mol dm-3) x Vol (dm3)

Question 7

What is 1 dm3 the same as?

Answer 7

1000cm3 or 1 litre.

Question 8

You may be asked to use more than one formula with the concentration formula in the exam e.g. mass, moles, mr equation.
How would you go about this?

Answer 8

1. no. of moles = Conc. x Vol (cm3) / 1000

2. Mass = no. of moles x Mr

Question 9

What is the ideal gas equation?

Answer 9

pV = nRT
p = Pressure (Pa)
V = Volume (m3)
n = no. of moles
R = Gas constant (8.31 J K-1 mol-1)
T = Temperature (Kelvin (K))

Question 10

How would you convert Pressure to Pa, Volume to m3 and temperature to K if they’re found in a different unit in the exam?

Answer 10

Pressure:
KPa--->Pa (x 1000)
MPa--->Pa (x 1000,000)
Volume: 
dm3--->m3 (/1000)
cm3--->m3 (/1000,000)
Temperature:
oC--->K (+273)

Question 11

Rearrange pV=nRT to make the following the subject of the formula:

1. n?
2. p?
3. V?
4. T?

Answer 11

1. n = pV/RT
2. p = nRT/V
3. V = nRT/p
4. T = pV/nR

Question 12

How do you balance an equation?

Answer 12

1. They have to have the same no. of each atom on both sides.
2. You can only change the front number.
3. You can use 1/2 to balance but you should only use it for diatomic molecules like O2, H2 or Cl2.

Question 13

How you write an ionic equation?

Answer 13

1. Start by writing a full, balanced equation for the reaction.
2. Then split any dissolved ionic species up into ions.
3. Finally take out any ions that appear on both sides of the equation.
4. Once you’ve written the ionic equation, check that the charges are balanced.

Question 14

How can you calculate masses? (Finding how much product you’ll get from a certain mass of a reactant)

Answer 14

1. Write out the balanced equation for the reaction.
2. Work out how many moles of the reactant you have.
3. Use molar ratio from the balanced equation to work out no. of moles of product that’ll be formed from this much reactant.
4. Calculate the mass of that many moles of product.

Question 15

Why is it useful to know how much gas a reaction will produce?

Answer 15

So you use a large enough apparatus or else there may be a huge bang.

Question 16

How would you approach a question which combines mass calculations and the ideal gas equation to calculate a gas volume?

Answer 16

1. The first three steps are the same as the ones for calculating masses.
2. Once you’ve found the no. of moles of product, put that number into the ideal gas equation.

Question 17

What does it mean if something dissolves/reacts in excess_______?

Answer 17

It means that reactant fully reacted.

Question 18

What is a neutralisation reaction?

Answer 18

When an acid reacts with an alkali to produce salt + water.

Question 19

What is purpose of a titration?

Answer 19

Involve neutralisation reactions to work out the concentration of an acidic or alkaline solution.

Question 20

Before you do titrations, what do you have to make up and what is it?

Answer 20

A standard solution is any solution that you know the exact concentration off. It involves dissolving a known amount of solid in a known amount of water to create a known conc.

Question 21

What can standard solutions also be referred to as?

Answer 21

Volumetric solutions.

Question 22

E.g. how would you make 250cm3 of a 2.00 mold dm-3 solution of Sodium Hydroxide?

Answer 22

1. Work out how many moles of sodium hydroxide you need using formula moles = concentration x volume
2. Now work out how many grams of sodium hydroxide you need using formula mass = moles x Mr
3. Place a weighing bottle on a digital balance + weigh out the required mass of solid approx. + tip into beaker.
4. Weigh weighing bottle that may still contain traces of solid + do: Mass of bottle with solid - mass of bottle.
5. Add distilled water to beaker + stir until all NaOH has dissolved.
6. Tip solution into 250cm3 volumetric flask using funnel to make sure all goes in.
7. Rinse beak, funnel, stirring rod with distilled water + add that to flask too-to make sure no solute is clinging to the beaker or rod.
8. Now top flask with more distilled water to correct volume. Make sure bottom of meniscus reaches line + when you get close to line add water drop by drop. (if you go over line you’ll have to begin again).
9. Stopper flask + rotate/turn up + down few times to make sure its all mixed well.
10. Now calc… exact conc. of your standard solution.

Question 23

What is an empirical formula?

Answer 23

Gives the smallest whole number ratio of atoms in a compound.

Question 24

What is a molecular formula?

Answer 24

Gives the actual number of atoms in a molecule.

Question 25

What is the molecular and empirical formula of Butane?

Answer 25

Molecular: C4H10 Empirical: C2H5

Question 26

What are the steps for calculating molecular formula if you know the empirical formula and the relative molecular mass of a compound?

Answer 26

1. Find empirical mass (just the relative mass of the empirical formula). 2. Do Relative molecular mass / empirical mass which tells you how many multiples of the empirical formula are in the molecular formula. 3. Multiply the empirical formula by that number to find the molecular formula.

Question 27

What are the steps for calculating empirical formulas from the percentages of different elements?

Answer 27

1. Assume you've got 100g of compound-you can turn percentages into masses + then work out how many moles of each element are in 100g using n = m/Ar 2. Then divide each no. of moles by smallest no. of moles. This gives you the ratio of the elements in the compound. 3. Apply the numbers from the ratio to the formula.

Question 28

What are the steps for calculating empirical formulas from the percentages of different elements if you're only given the percentage of some of the elements in the compound?

Answer 28

You have to work out the percentages of the others. 1. Minus from 100 which ever percentages for elements you had been given. 2. Do mass / Ar for each element. 3. Divided each no. of moles by the smallest no. of moles. This tells you the molar ratio. 4. All the numbers in empirical formula need to be whole numbers so you need to multiply by a certain number to get them whole if they previously weren't. 5. Apply them to the formula.

Question 29

What are the steps for calculating empirical formulas from the percentages of different elements if you're only given the percentage of some of the elements in the compound?

Answer 29

You have to work out the percentages of the others. 1. Minus from 100 which ever percentages for elements you had been given. 2. Do mass / Ar for each element. 3. Divided each no. of moles by the smallest no. of moles. This tells you the molar ratio. 4. All the numbers in empirical formula need to be whole numbers so you need to multiply by a certain number to get them whole if they previously weren't. 5. Apply them to the formula.

Question 30

Define theoretical yield?

Answer 30

The mass of product that should be formed in a chemical reaction. Assumes no chemicals are 'lost' in process.

Question 31

What are the steps for calculating theoretical yield?

Answer 31

1. Work out how many moles of reactant you have using n = m / Ar 2. Use equation to work out how many moles of product you would expect that much reactant to make using mole ratio. 3. Now calc... the mass of that many moles of product.

Question 32

Define 'hydrated'?

Answer 32

'Hydrated' means that the crystals have a bit of water left in them. That's what the dot in .H2O means e.g. (NH4)2Fe(SO4)2.6H2O means each molecule of (NH4)2Fe(SO4)2 in the crystal is surrounded by sic water molecules.

Question 33

Define percentage yield?

Answer 33

For any reaction, the actual mass of product will always be less than the theoretical yield.

Question 34

What is the equation for percentage yield?

Answer 34

Actual yield / Theoretical yield ( x 100 )

Question 35

Give some reasons why the actual mass of product will always be less than the theoretical yield?

Answer 35

1. Not all starting chemicals react fully. 2. Some chemicals are 'lost' e.g. solution left on the filter paper. 3. Lost during transfers between containers. 4. Lost when forming other products you don't want in side reactions.

Question 36

How would you approach a question if you're not given the theoretical yield when being asked to calculate the percentage yield?

Answer 36

You need to work out theoretical yield. 1. Use n = M / Mr to find the moles of the reactant you have been given in the question. 2. Work out how many moles of product you would expect to make using molar ratio. 3. Use this in M = n x Mr to calculate the mass of that many moles of product. This is the theoretical yield. 4. Now put these numbers into the percentage yield formula.

Question 37

What is atom economy?

Answer 37

A measure of the proportion of reactant atoms that become part of the desired product (rather than by-products) in the balanced chemical equation.

Question 38

What are the advantages and disadvantages of percentage yield?

Answer 38

Its based on how much of product is lost due to incomplete reactions or losses during collection and purification. +ves: Useful info about how wasteful process is. -ves: Doesn't measure how wasteful reaction itself is (reaction itself could have 100% yield but still be v. wasteful if lots of atoms from reactants turn into by-products rather than desired products.

Question 39

Define Atom economy?

Answer 39

A measure of the proportion of reactant atoms that become part of the desired product (rather than by-products) in the balanced chemical equation.

Question 40

What are the economic advantages of high atom economy?

Answer 40

Companies in chemical industry often choose high AE reactions. 1. Companies using high AE will make more efficient use of its raw materials. Low AE process mean using large quantities of raw materials to make small amount of desired products + lots of by-products. Waste of £. 2. High AE process also means company has less waste to deal with. Less £ spent on separating desired + waste products. Any waste produced needs to be safely disposed of which costs £.

Question 41

What are the environmental and ethical advantages of high atom economy?

Answer 41

1. Processes that use fewer raw materials + produce less waste better for enviro... + business (makes sense to use raw material in limited supply, efficiently to last longer). Less waste produced is better for enviro... as chemicals are often harmful for it. Difficult to dispose in way that minimises their harmful effects. 2. High AE more sustainable than low AE. Sustainable-Using up as little of earth's resources as you can + not putting loads of damaging chems into enviro... for future. 3. Good for society if chem companies find easier, cheaper ways to mass produce medicines + other useful chems as this may mean products can be £ for lower prices + be more available to more people.

Question 42

What is the formula for calculating atom economy?

Answer 42

Mr of desired product / sum of Mr of all reactants ( x 100) ) = % Atom Economy