5: Formulae, Equations and Amounts of Substance

Question 1

What is molecular formula

Answer 1

The formula that shows the number and type of each atom in a molecule (e.g. C2H4O2)

Question 2

What is empirical formula

Answer 2

The simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound

Question 3

How can empirical formula be calculated from mass

Answer 3

Question 4

How can empirical formula be calculated by percentage

Answer 4

Question 5

How can molecular formula be calculated from empirical formula

Answer 5

• Use empirical formula to calulate the relative empirical mass (e.g. C4H10 = (12x4) + (1x 10)
• Divide the relative formula mass by the relative empirical mass
• Multiply the elements by the number produced

Question 6

What is water of crystallisation

Answer 6

• When some compounds can form crystals which have water as part of their structure
• It is a hydrated compound
• The compound without water is called anhydrous
• The conversion of anhydrous compounds and hydrated compounds is reversible by heating hydrated salts or dissolving the anhydrous ones

Question 7

How to calculate the water of crystallisation

Answer 7

Question 8

What is the ideal gas equation, and what units are used

Answer 8

Question 9

How to convert degrees Celsius to Kelvin

Answer 9

+273

Question 10

How to convert cm3 to m3

Answer 10

Multiply by 10^-6

Question 11

How to convert dm3 to m3

Answer 11

Multiply 10^-3

Question 12

What is the equation for percentage mass of an element

Answer 12

Question 13

What to remember when balancing equations

Answer 13

• Don’t change any formula
• Put balancing number in front of formula
• Remener to put state symbols

Question 14

What to remember when writing ionic equations

Answer 14

• Remember to balance the original equation
• Spectator ions don’t take part in the equation
• Only the ions that are reacting
• Spectator ions can only be cancelled if they are the same on both sides (balanced)

Question 15

What is a precipitation reaction

Answer 15

Two aqueous solutions reacting to form products which include one solid

Question 16

What is the Avogadro constant

Answer 16

• The number of particles equivalent to the relative atomic mass or molecular mass of a substance
• 6.02 x 10^23 g mol-1

Question 17

What is the equation for moles

Answer 17

Question 18

How can Avogadro’s constant be used to calculate number of particles

Answer 18

Number of particles = Avogadro constant x moles of substance

Question 19

How can Avogadro’s constant be used to calculate mass of atoms

Answer 19

Mass of atom (g) = relative atomic mass of atom/avogadro constant

Question 20

How can Avogadro’s constant be used to calculate mass of molecules

Answer 20

Mass of molecule (g) = relative formula mass of molecule/avogadro constant

Question 21

What is the equation to find concentration

Answer 21

Concentration = Moles/Volume

Question 22

How to calculate volume

Answer 22

Volume = Number of moles x 24dm3

Question 23

What are random errors and how can they be affected

Answer 23

• Errors that occur by chance and pull the result away from expected in either direction (high or low)
• Person reading scales badly
• Instruments scaling system
• Changes in environment

Question 24

How can the effect of random errors be reduced

Answer 24

• Repeating the experiment
• Using the mean average of data
• Ignoring anomalies

Question 25

What are systematic errors with examples

Answer 25

- Errors that occur as a result of a faulty or poorly designed experimental procedure and pull the result away from expected in the same direction (always high or always low) - Parallax error, bungs not being efficient

Question 26

How can the effects of systematic errors be reduced

Answer 26

- Setting up the experiment with new equipment - Having the experiment done multiple times by different people with different equipment

Question 27

What is the equation for percentage uncertainty

Answer 27

Percentage uncertainty = (absolute uncertainty/measured value) x100

Question 28

What is the equation for percentage error

Answer 28

Percentage error = (Estimated number - actual number / actual number) x100

Question 29

What is the equation for percentage yield

Answer 29

Percentage yield = (actual yield/theoretical yield) x100

Question 30

What are the reason that the percentage yield is under/over 100%

Answer 30

- Under: reaction didn’t go to completion, products were lost, other reactions take place simultaneously - Over: Preparing crystals that may be dry still when weighing, impurities

Question 31

What is the equation for atom economy

Answer 31

Question 32

What is the atom economy for addition reactions

Answer 32

It will always be 100%, as all atoms were used to make the desired product

Question 33

What is atom economy

Answer 33

How many of the atoms used in the reaction became the desired product, instead of waste

Question 34

What do titrations measure

Answer 34

The concentration of an unknown solution

Question 35

How is a standard solution for a titration prepared

Answer 35

- Weigh out a precise amount of solid - Add to a small volume of water to dissolve it - Transfer to a volumetric flask - Rinse the beaker with distilled water and add to the flask - Make the solution up to the maximum mark (bottom of meniscus) of the flask with distilled water

Question 36

Describe the process of a titration

Answer 36

- Measure a known volume of a solution with a volumetric pipette and put it in a conical flask - The other solution is put in the burette (usually to 0.00cm3) - Indicator is added to the solution in the conical flask - Remove the funnel from the burette and measure the initial volume - Add small amounts of solution into the conical flask, and swirl - Near the end point, place the conical flask on a white tile and add the solution from the burette drop wise - Once the colour change has occurred, measure the final volume - Repeat multiple times

Question 37

How to obtain results from titration data

Answer 37

- Subtract the initial volume from the final volume - Select the data points that are concordant (not including the rough run)

Question 38

What are concordant results

Answer 38

Data within 0.1 cm3 of each other

Question 39

Why is the percentage uncertainty double for titration volumes

Answer 39

An initial and final volume were taken, so the percentage certainties from both are considered