5. Formulae, Equations and Amounts

Question 1

Mole

Answer 1

the amount of a substance that contains the same number of particles found in 12 g of 12 C.

Question 2

Avogadro’s constant

Answer 2

The number of atoms in exactly 12 g of 12 C (6.02 x 10 23 mol − 1)

Question 3

Empirical formula

Answer 3

Smallest whole number ratio of atoms of each element in a compound

Question 4

Molecular formula

Answer 4

The actual number of atoms of each element in a molecule

Question 5

Spectator ions

Answer 5

Ions that do not take part in the reaction

Question 6

Displacement reaction

Answer 6

More reactive element reacts to take place of less reactive element in a compound

Question 7

Precipitation reaction

Answer 7

produces an insoluble solid

Question 8

Precision

Answer 8

Refers to how close to each other are the values obtained in an experiment

Question 9

Accuracy

Answer 9

Refers to how close these values are to the actual value

Question 10

Concordant results

Answer 10

Results that lie close to each other. In titration, these are titres that usually lie within +/ − 0.20 cm 3

Question 11

Random errors

Answer 11

They occur when conditions are varied in an unpredictable manner

Question 12

Systematic errors

Answer 12

Errors which are constant when you repeat an experiment. They usually are a result of the apparatus used

Question 13

Percentage uncertainty

Answer 13

(Uncertainty/Reading) x 100%

Question 14

What is the multiplier of precision error for a balance

Answer 14

x2

Question 15

What is the multiplier of precision error for a burette

Answer 15

x2

Question 16

What is the multiplier of precision error for a pipette

Answer 16

x1

Question 17

What is the multiplier of precision error for a volumetric flask

Answer 17

x1

Question 18

What is the equation for experimental error

Answer 18

(real answer - experiment answer) / real answer x 100

Question 19

What is the conclusion if experimental error is less than the percentage error for an apparatus

Answer 19

the results are accurate

Question 20

Percentage yield

Answer 20

(Actual yield/Theoretical yield) x 100%

Question 21

What is actual yield

Answer 21

the mass of product obtained experimentally

Question 22

What is theoretical yield

Answer 22

the maximum mass that should have been produced based on the moles of the limiting reactant

Question 23

Give five reasons for % yield not being 100%

Answer 23

1) reactants may not be totally pure
2) some of the product may have been lost during the transfer of chemicals between reaction vessels
3) side reactions may convert products into unwanted by-products
4) slowing rate of reaction might mean not all reactants are converted
5) may be a reversible reaction where equilibrium is established

Question 24

Atom economy

Answer 24

Measure of the proportion of reaction atoms that become part of the desired product in the balanced chemical equation

Question 25

Atom economy equation

Answer 25

(Molar mass of desired product/Total molar mass of all products ) x 100%

Question 26

Describe the heating to dryness practical

Answer 26

1) weigh sample of hydrated salt into pre-weighed crucible 2) place lid loosely on crucible and heat for 5 minutes to remove the water of crystallisation 3) allow crucible to cool, remove lid, and reweigh the crucible with its contents

Question 27

What is the role of the lid in the heating to dryness practical

Answer 27

prevents any salt from escaping during heating

Question 28

How do we ensure all water is removed in the heating with dryness practical

Answer 28

1) after weighing, reheat for 2 minutes 2) cool and reweigh 3) if all the water has been lost, the mass value won't have changed 4) repeat until a constant is achieved

Question 29

Describe how to prepare a standard solution

Answer 29

1) accurately weigh an appropriate mass of solute by weighing by difference 2) dissolve all of the solute in the beaker by adding 75-100cm3 of distilled water and stir using a glass rod 3) rinse out the volumetric flask with distilled water and pour the solution into the volumetric flask using a funnel 4) rinse the inside of the beaker repeatedly and pour these washings into the volumetric flask 5) use a dropping pipette to make the solution up to the mark, to ensure that the base of the meniscus is on the mark 6) place a lid on the volumetric flask and invert the flask multiple times

Question 30

Describe how to carry out a titration to find the unknown concentration of a solution

Answer 30

1) rinse the burette with distilled water 2) fill the burette with the solution we are determining the concentration of and run solution through the tap to ensure that the tap is full 3) accurately pipette out 25cm3 of standard solution into a distilled conical flask 4) add 3-4 drops of indicator to the conical flask and place on a white tile 5) continuously swirl the conical flask whilst slowly adding solution from the burette until the solution permanently changes colour 6) determine the volume of solution added to the conical flask 7) repeat until concordant results are reached