🟣 Topic 5(?): Calculations

Question 1

How do you calculate percentage yield?

Answer 1

Percentage yield = (actual yield / theoretical yield) x 100

Question 2

What is theoretical yield?

Answer 2

The maximum mass of product that can be formed from a given amount of a reactant

Question 3

What is the actual yield?

Answer 3

The actual mass of product formed during a reaction

Question 4

How do you calculate the theoretical yield?

Answer 4

Calculated from balanced equation - Find Mr of given reactant and wanted product. Then convert that to the mass.

Question 5

Why is the percentage yield always less than 100%? (Why is the actual yield less than the theoretical yield?)

Answer 5

• incomplete reaction
• practical losses during experiment
• competing side equations

Question 6

What is an example of a competing side equation?

Answer 6

Carbon monoxide or carbon being formed if not enough oxygen

Question 7

Give an example of practical losses during experiment

Answer 7

Some liquid left on walls on container when transferring liquid from one to the other

Question 8

When will there be an incomplete reaction?

Answer 8

When the reaction reaches equilibrium

Question 9

What is atom economy?

Answer 9

It shows the percentage, by mass, of useful products formed in a reaction

Question 10

How do you calculate atom economy?

Answer 10

Atom economy = (relative formula mass of the useful product / sum of relative formula masses of all the reactants) x 100

Question 11

When is atom economy 100%?

Answer 11

When all atoms from reactants are present in product

Question 12

When is atom economy less than 100%?

Answer 12

When all atoms from reactants are not present in (desired) product

Question 13

Explain a reason for why a low atom economy is a disadvantage in a manufacturing process

Answer 13

Lots of waste products and they may be toxic or corrosive so they will be difficult or expensive to dispose of

Question 14

State how the atom economy can be improved

Answer 14

• find uses for waste products

- find an alternative reaction pathway which makes less by-products

Question 15

If a reaction occurs in two steps and each step has a percentage yield of 80%, what is the overall percentage yield?

Answer 15

(80/100) x (80/100) = 0.64

0.64 x 100 = 64%

Question 16

Explain, in terms of atom economy and percentage yield, the advantages of a three step process compared with a six step process

Answer 16

• fewer steps are likely to result in a higher overall atom economy so there will be fewer waste products to dispose of - this will reduce the cost
• fewer steps will also give a higher overall percentage yield so less of the product will be lost

Question 17

Suggest some other factors that will influence the choice of a reaction pathway

Answer 17

• rate of reaction
• equilibrium position
• usefulness of the by-products

Question 18

What is 1 mole?

Answer 18

6.02 x 10^23 particles/ atoms/ molecules

Question 19

What is another name for 1 mole?

Answer 19

Avogadro‘s constant

Question 20

How do you calculate the number of particles/ atoms/ molecules?

Answer 20

No. of particles/ atoms/ molecules = no. of moles x Avogadro‘s constant
N = n x Av

Question 21

How do you calculate the mass (grams)? (Using moles)

Answer 21

Mass (grams) = no. of moles x relative mass (from periodic table)
m = n x Mr

Question 22

How do you calculate the number of moles? (Two ways)

Answer 22

No. of moles = no. of particles/atoms/molecules / Avogadro‘s constant
No. of moles = mass (grams) / relative mass (from periodic table)

Question 23

What is there a conservation of in reactions?

Answer 23

Mass

Question 24

What is a closed system?

Answer 24

No new substances are added or removed (eg. a gas escaping)

Question 25

How do you calculate the empirical formula?

Answer 25

Mass (given with question)
Relative atomic mass (from periodic table)
Divide mass by RAM
Divide answers by Smallest number to find simplest ratio
Formula (empirical)
(If any decimals at the end, times until both are whole numbers)

Question 26

Why would an experiment to find the empirical formula of magnesium oxide not be reliable?

Answer 26

There won’t be a perfect relationship between the magnesium and oxygen as some oxygen could have been lost when the lid was lifted to allow more oxygen in to react

Question 27

What is an empirical formula?

Answer 27

The simplest number of atoms present in a compound

Question 28

How do do calculate the Rf value in chromatography?

Answer 28

Rf = distance moved by substance / distance moved by solvent

Question 29

What should the answer for the Rf value always be and what are the units?

Answer 29

A decimal and there are no units

Question 30

How do you calculate concentration (two ways) and what are the units?

Answer 30

Concentration (g/dm^3) = mass (g) / volume (dm^3)
OR
Concentration (mol/dm^3) = moles (mol) / volume (dm^3)

Question 31

How do you change cm^3 to dm^3?

Answer 31

Divide by 1000

Question 32

How would you change the concentration of something from g/dm^3 to mol/dm^3?

Answer 32

Concentration in mol/dm^3 = (concentration in g/dm^3) / (relative formula mass of solute)

Question 33

What is the value of molar volume always?

Answer 33

24dm^3

24,000cm^3

Question 34

How do you calculate the volume of gas?

Answer 34

Volume of gas (cm^3/dm^3) = no. of moles x molar volume

Question 35

What do you have to remember when calculating the mean titre?

Answer 35

• don’t use results from first titre (rough one)

- only use concordant results within 0.2cm^3 of each other

Question 36

When finding a limiting reactant (or the one in excess), what do you need to find out to compare them and how?

Answer 36

The number of moles using the equation: no. of moles = mass / relative mass

Question 37

What do the big numbers in front of the formulas in an equation show?

Answer 37

The ratio of moles between the elements

Question 38

What is stoichiometry used for?

Answer 38

To find balanced equations

Question 39

What are the steps of stoichiometry?

Answer 39

• find number of moles (n = m / Mr)
• divide by smallest to find ratio
• (if needed, times to find whole number)