1.2 Amounts of Substance

Question 1

What is the definition of empirical formula?

Answer 1

The SIMPLEST whole number ratio of atoms in a compound

Question 2

What is the definition of molecular formula?

Answer 2

The actual number of atoms of each element in a compound

Question 3

Why is the actual molecular mass not need to be known exactly in an empirical formula question?

Answer 3

Because the Mr is an average

Question 4

How do you calculate empirical formula?

Answer 4

If given in percentage treat percentage as mass
Mass / Mr
Divide through by smallest number

Question 5

How do you calculate moles of hydration?

Answer 5

If percentage treat as mass

Question 6

What is the definition of relative atomic mass?

Answer 6

Average mass of an element compared to 1/12 mass of a carbon 12 atom

Question 7

What is avogadros constant?

Answer 7

6.022 x 10 ^23The number of particles in a mole

Question 8

How to calculate the number of particles?

Answer 8

Mass / Mr
Ans x avogadros constant

Question 9

What is the definition of a mole?

Answer 9

The amount of a substance with the same number of molecules in 12 g of carbon 12

Question 10

What is the mole equation with a mass in grams?

Answer 10

Moles = mass/mr

Question 11

Equation to calculate number of moles of a gas at normal conditions?

Answer 11

Moles= volume/24000

Question 12

How to calculate the number of moles in a solution?

Answer 12

Moles= concentration x volume

Question 13

How do you work out how much of one product is needed to react with a known mass?

Answer 13

Balance equation
Work out ratio of moles
Work out mole of known substance
Deduce moles of unknown substance
Moles x Mr = answer

Question 14

How do you find out the concentration from a titration?

Answer 14

• Balance equation
• Calculate number of moles of substance that’s got two parts known
• Using ratio of moles work out the moles of substance with unknowns
• Use equation moles = concentration x volume

Question 15

What is the ideal gas equation?

Answer 15

PV = nRt

Pressure (Pa) x volume (m^3)= number of moles (mol) x gas constant (mol^-1k^-1) c temperature (k)

Question 16

How do you turn Celsius in to kelvin?

Answer 16

You add 273

Question 17

How do you turn dm3 into m3?

Answer 17

Divide by thousand

Question 18

What is the definition of atom economy?

Answer 18

Percentage of reactants that become useful products. Shows how efficient a reaction is.

Question 19

What is the atom economy formula?

Answer 19

Mr of desired product/sum of mr of all products x 100

Question 20

What is the percentage yield equation?

Answer 20

Actual yield/ theoretical yield x 100

Question 21

Why is actual yield never 100%?

Answer 21

• Mass lost in transfer operations
• Reversible reactions
• Unwanted side reactions

Question 22

Why are less efficient reactions still used?

Answer 22

• One may be more environmentally friendly than the other
• less energy may be used
• waste product may be sellable

Question 23

How do you do a reacting mass calculation?

Answer 23

Balance equation to find ratio of moles
Calculate moles of known mass
Deduce number of moles of unknown amount
Use moles = mass/mr

Question 24

What equipment is used to measure a set volume of solution?

Answer 24

Bulb Pippette

Question 25

What piece of equipment is used to prepare a standard solution?

Answer 25

Volumetric flask

Question 26

What piece of equipment measures variable volumes of solution?

Answer 26

Burette

Question 27

What piece of equipment has sides to reduce loss of solution through splashing?

Answer 27

Conical flask

Question 28

How is percentage uncertainty calculated?

Answer 28

Uncertainty / volume of apparatus x 100

Question 29

Why is limited amounts of indicator used in a titration?

Answer 29

Indicator is a weak acid
Make titre less accurate

Question 30

Why is a funnel left in the top of a Burette during a titration incorrect?

Answer 30

Drops left in funnel could drop into Burette making the volume left in Burette incorrect

Question 31

Why is the tip of the burette not being filled up incorrect in a titration?

Answer 31

Less than the stated amount in the burette so the volume is incorrect

Question 32

Why is it not good if the burette he not been washed out with acid?

Answer 32

Water may still be in burette which will affect pH

Question 33

What is the colour change of methyl orange?

Answer 33

Yellow to Pink

Question 34

Why is a small titration problematic and what improvements can be done?

Answer 34

Percentage error is high
Increase concentration of reagent
Increase mass

Question 35

What should all calculations be rounded to unless otherwise stated?

Answer 35

3 significant figures

Question 36

What happens in a back titration?

Answer 36

The concentration of a reactant is determined by reacting it with an excess of a known amount of a reagent

Question 37

When are back titrations used?

Answer 37

When the reactant is insoluble

Question 38

What are the steps to find percentage mass of a substance by back titration?

Answer 38

Write out and balance equation
Find out number of moles of reagent reactant was titrated with
Deduce number of moles of unknown based on molar ratio
Calculate number of moles in entire flask before titration (e.g. if 25cm3 was titrated from 250cm3 x by 10)
Calculate mass using moles = mass/ mr
Divide mass of reactant over overall substance