3.1.2 Amount of Substance Knowledge

Question 1

define Avogadro’s Constant

Answer 1

number of atoms in 12g of 12C

Question 2

how do you calculate the number of particles?

Answer 2

Number of Particles = Moles x Avogadro’s Constant

Question 3

what is relative atomic mass?

Answer 3

average mass of one atom of element / 1/12th the mass of a C12 atom

Question 4

how do you calculate relative formula mass?

Answer 4

average mass of one molecule / 1/12th the mass of a C12 atom

Question 5

what is the difference between relative formula and relative molecular mass?

Answer 5

Relative formula mass is used for ionic compounds because, strictly speaking, they don’t exist as simple molecules. Relative molecular mass and relative formula mass can both be calculated and used in the same way.

Question 6

how do you calculate the number of moles?

Answer 6

moles (mol) = mass (g) / molar mass (g/mol)

Question 7

how do you calculate concentration?

Answer 7

concentration (mol dm^-3) = moles (mol) / volume (dm^3)

Question 8

how many cm^3 are in dm^3?

Answer 8

1000

Question 9

what is the purpose of a titration?

Answer 9

to use one chemical solution of a known concentration (a standard solution) to determine the concentration of another by reacting them together

Question 10

what are concordant results?

Answer 10

within 0.1cm^3 of each other

Question 11

what are the Steps to make up a Standard Solution?

Answer 11

• Stage 1: transfer known mass of solid
  
  • weigh sample bottle containing solid on 2 d.p. balance
  • transfer to beaker and reweigh empty sample bottle
  • record difference mas; this is = mass of solid
• Stage 2: dissolve in water
  
  • add distilled/deionised water
  • stir with glass rod or swirl
  • until all solid has dissolved
• Stage 3: Transfer, washing, and agitation
  
  • Transfer to volumetric/graduated flask using funnel with washings
  • make up to 250cm^3/mark with water
  • shake/invert/mix

Question 12

what 3 things can be done to increase the accuracy of a titration?

Answer 12

• Remove any air bubbles from the burette tap
• Remove the funnel used to fill the burette before starting the titration
• During the titration, swirl the conical flask and rinse the walls of the conical flask with distilled water

Question 13

why should you Remove any air bubbles from the burette tap in a titration?

Answer 13

otherwise, the volume of the air bubble will be counted in the burette reading, resulting in a greater titre

Question 14

why should you Remove the funnel used to fill the burette before starting a titration?

Answer 14

to prevent any further liquid from entering the burette

Question 15

why should you swirl the conical flask and rinse the walls of the conical flask with distilled water during a titration?

Answer 15

• to ensure all reactants mix and no reagent is left unreacted on sides of flask.
• Doesn’t change the number of moles in conical flask, so has no adverse effect on titration.

Question 16

how do you calculate percentage error?

Answer 16

% error = (error of equipment / amount measured) * 100

Question 17

what is yield?

Answer 17

amount of product made in a reaction

Question 18

how do you calculate percentage yield?

Answer 18

(Actual Yield / Theoretical Yield) * 100

Question 19

why, When using a chemical reactant to prepare a substance, is it desirable to have a high percentage yield?

Answer 19

this increases the amount of product formed

Question 20

how do you improve percentage yield?

Answer 20

improve experimental technique

Question 21

WHAT is the ideal gas equation?

Answer 21

PV = nRT where:
- P = pressure, Pa
- V = volume, m^3
- n = moles, mol
- R = constant, 8.32 JK^-1mol^-1
- T = temperature, K

Question 22

how much is 0 celsius in kelvin?

Answer 22

273K

Question 23

what are the Factors that affect pressure?

Answer 23

• number of moles, n
• temperature, t
• volume, v

Question 24

what is the empirical formula?

Answer 24

the simplest whole number ratio of atoms of each element in a compound

Question 25

what is the molecular formula?

Answer 25

the actual number of atoms of each element in a compound

Question 26

what is water of crystallisation?

Answer 26

refers to water molecules incorporated into a compound, in fixed proportions

Question 27

how do you convert from cm^3 to dm^3?

Answer 27

divide by 1000

Question 28

how do you convert from cm^3 to m^3?

Answer 28

divide by 1,000,000

Question 29

how do you convert from dm^3 to m^3?

Answer 29

divide by 1000

Question 30

how many g is 1000mg?

Answer 30

1g

Question 31

how many mg is 1g?

Answer 31

1000mg

Question 32

how many g is 1 kg?

Answer 32

1000g

Question 33

how many kg is 1000g?

Answer 33

1kg

Question 34

how many kg is 1 tonne?

Answer 34

1000kg

Question 35

how many tonnes is 1000kg?

Answer 35

1 tonne

Question 36

how many signficant figures should you give your answers to?

Answer 36

The same number of sig figs as the data in the question. If there is a mixture, use the smallest.

Question 37

How can you improve the accuracy of measuring mass afetr heating in a crucible?

Answer 37

use a lid

Question 38

how does using a lid improve the accuracy of measuring mass after heating in a crucible?

Answer 38

it prevents loss of solid from the crucible, but should ne loose fitting to allow gas to escape

Question 39

why should the crucible be dry when measuring mass after heating in a crucible?

Answer 39

A wet crucible would give an innacurrate result; it would cause mass loss to be too large, as the water would be lost when heating

Question 40

why should large amounts of hydrated CaSO4 not be used when heating in a crucible?

Answer 40

the decomposition is likely to be incomplete

Question 41

why should small amounts of hydrated CaSO4, such as 0.1g, not be used when heating in a crucible?

Answer 41

the percentage uncertainties in weighing will be too high

Question 42

how do you convert from concentration measured in mol dm^-3 to g dm^-3?

Answer 42

multiply by the Mr of the substance

Question 43

why is using a volumetric pipette more accurate than a measuring cylinder?

Answer 43

it has a smaller uncertainty

Question 44

what more accurate piece of equipment could be used instead of a measuring cylinder?

Answer 44

volumetric pipette

Question 45

how do you calculate new diluted concentration?

Answer 45

original concentration * (original volume / new diluted volume)

Question 46

what hazard do dilute acid and alkalis pose?

Answer 46

irritants

Question 47

what safety precaution should be taken against acids and alkalis?

Answer 47

wear goggles

Question 48

what hazard do strong acid and alkalis pose?

Answer 48

corrosive

Question 49

what safety precaution should be taken against flammable substances?

Answer 49

keep away from naked flames

Question 50

what safety precaution should be taken against toxic substances?

Answer 50

- wear gloves - avoid skin contact - wash hands after use

Question 51

what safety precaution should be taken against oxidising substances?

Answer 51

keep away from flammable/easily oxidised materials

Question 52

what are some potential errors in using a gas syringe?

Answer 52

- gas escapes before bung inserted - syringe sticks - some gases, like CO2 or SO2, are soluble in water so the true amount of gas is not measured

Question 53

what is the relationship between the volume of a gas and the number of molecules it has?

Answer 53

equal volumes of any gases measured under the same conditions of temperature and pressure contain equal numbers of molecules

Question 54

what is the volume of 1 mole of any gas at RTP?

Answer 54

24dm^3

Question 55

why is a high percentage yield beneficial?

Answer 55

because it means there has been an efficient conversion of reactants to products

Question 56

why is a high percentage atom economy beneficial?

Answer 56

so that the maximum mass of reactants ends up in the desired product, minimising the amount of by-product

Question 57

why are concordant results used in titrations?

Answer 57

it means that the results are accurate and repeatable, and the titration technique is good/consistent

Question 58

why is a conical flask used in preference to a beaker during titrations?

Answer 58

it is easier to swirl the mixture in a conical flask without spilling the contents

Question 59

why should you only add a few drops of indicators in titrations?

Answer 59

they are generally weak acids, so if too much is added it will affect the titration result

Question 60

what is it necessary to consider when titrating a mixture to work out the concentration of an active ingredient?

Answer 60

if the mixture contains other substances that have acid base properties

Question 61

why, in quality control, is it necessary to do titrations/testing on several samples?

Answer 61

as the amount/concentration of the chemical being tested may vary between samples

Question 62

what compounds are good for neutralising acid spills?

Answer 62

sodium hydrogen carbonate (NaHCO3) and calcium carbonate (CaCO3)

Question 63

why are sodium hydrogen carbonate (NaHCO3) and calcium carbonate (CaCO3) good for neutralising acid spills?

Answer 63

they are not corrosive and will not cause a hazard if used in excess

Question 64

how can you decrease apparatus uncertainties?

Answer 64

decrease the sensitivity uncertainty by using apparatus with a greater resolution or increase the size of the measurement made

Question 65

how do you reduce the percentage uncertainty in a burette reading?

Answer 65

make the titre a larger volume

Question 66

how can you make the titre a larger volume in a burette reading?

Answer 66

increase the volume and concentration of the substance in the conical flask, or decrease the concentration of the substance in the burette

Question 67

how can you decrease the % uncertainty in weighing a solid?

Answer 67

- use a balance that measures to more decimal places - use a larger mass - weigh a sample before and after addition and calculate the difference

Question 68

what does it mean if the % uncertainty due to the apparatus is less then the % difference between the actual value and the calculated value?

Answer 68

there is a discrepancy in the result due to other errors

Question 69

what does it mean if the % uncertainty due to the apparatus is more then the % difference between the actual value and the calculated value?

Answer 69

there is no discrepancy and all the differences between values can be explained by the sensitivity of the equpiment