5 - Formulae, Equations & Amounts Of Substances

Question 1

What are the two types of errors and how can they be avoided?

Answer 1

• Systematic - often caused by equipment or procedural errors, continued across all results. To avoid, use better equipment
• Random - often caused by human error (e.g. Reading burette incorrectly), varying between results. Overcome by repeating experiments (improving accuracy and removing outliers).

Question 2

In titrations, what are 3 possible errors as a result of poor technique?

Answer 2

• Not thoroughly shaking volumetric flask after topping up to 250cm^3
• Getting air below the tap in the burette
• Getting air in the stem of pipettes
• Dripping solution from burette too quickly, overshooting the end point
• Not swirling the flask after each drop of solution from the burette is added

Question 3

What is the formula for percentage uncertainty?

Answer 3

(Uncertainty/Difference in readings) x 100

Question 4

How do you work out the uncertainty for a certain piece of equipment?

Answer 4

Half the smallest increment the equipment is measurable to - e.g. A burette with increments of 0.1cm^3 has an uncertainty of +/- 0.05cm^3

Question 5

What are two ways in which % uncertainty can be minimised?

Answer 5

• Use more precise equipment with lower uncertainties

* Use larger volumes of solution

Question 6

How do you find the actual total uncertainty in a titration reaction?

Answer 6

1. ) Find individual % uncertainties for each bit of equipment
2. ) Add the percentages together
3. ) Find that percentage of the end result of the titration

Question 7

What is the formula for percentage yield?

Answer 7

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

Question 8

What is atom economy?

Answer 8

The proportion of reactants that become useful products

Question 9

What is the formula for atom economy?

Answer 9

Atom Economy = (Mass of desired product/Sum of the masses of all products) x 100

Question 10

What is the atom economy in addition reactions, where only one product is formed? And why?

Answer 10

The atom economy is 100%, because there are no by-products

Question 11

What is a mole?

Answer 11

The amount of a substance containing the same amount of particles as in 12g of carbon 12

Question 12

How do you work out the number of particles?

Answer 12

Moles x Avogadro constant

Question 13

How do you work out mass?

Answer 13

Mass = moles x RMM

Question 14

How do you work out concentration?

Answer 14

Conc. = moles/volume (dm^3)*

• if in cm^3, Divide by 1000

Question 15

How do you work out the empirical formula from percentage composition?

Answer 15

1. ) calculate miles of elements: percentage/mass
2. ) Divide all by smallest number to get ratio
3. ) use ratio to find how many atoms of each element there are, giving empirical formula

Question 16

How do you work out the empirical formula from combustion masses?

Answer 16

1. ) Find moles using masses
2. ) multiply moles by amount of element in compound
3. ) Divide moles by smallest to get ratio

Question 17

How can you use the empirical mass to work out the molecular?

Answer 17

Molar mass/empirical mass = n

Multiply compound by n to get Molecular mass

Question 18

What are displacement reactions?

Answer 18

A more reactive element replacing less reactive element in a compound

Question 19

What does an acid/base reaction form?

Answer 19

Acid + base -> salt + water

Question 20

What is a precipitation reaction?

Answer 20

2 aqueous compounds reacting to form solid precipitate

Question 21

What is the gas triangle?

Answer 21

Volume = moles x 24,000cm^3

Question 22

What experimental setups can you use to collect gas?

Answer 22

• gas syringe and conical flask

* conical flask and inverted measuring cylinder in water

Question 23

What is the ideal gas equation?

Answer 23

pV = nRT

pressure x volume = moles x 8.31 x temperature

Question 24

What is the method for an acid-base titration?

Answer 24

1. ) rinse burette and pipette and conical flask with distilled water
2. ) fill burette with standard solution
3. ) open tap to remove air bubbles. Read to 0.05cm3
4. ) Add indicator to beaker with base or acid in
5. ) do rough titration where burette solution dropped quickly into beaker
6. ) repeat at least 2 more times with dripping burette to get concordance titres. Average these to get mean titre

Question 25

What are 2 examples of indicators and what colours are they?

Answer 25

• methyl orange - red in acid, yellow in alkali . Orange at end point in titration
• phenolphthalein - colourless in acid, purple in alkali. Pink at end point

Question 26

Before preparing a standard solution, what 3 things should you ensure the acid or base is?

Answer 26

• obtainably pure
• not absorbing moisture or CO2 from air
• not losing water of crystallisation to the air

Question 27

What 3 compounds can’t be used to standard solutions?

Answer 27

• sulphuric acid (not obtainably pure, absorbs CO2 or moisture from air)
• sodium hydroxide/bases (absorb moisture/CO2)
• hydrated sodium carbonate (lose water of crystallisation to air)