Topic 5 Formulae, Equations and amounts of a substance

Question 1

What is a mol?

Answer 1

It is a way of measuring the amount of a substance in chemistry
contains 6.02 x 10^23 atoms/molecules

Question 2

How do you calculate the number of particles?

Answer 2

Number of particles = Avogadro’s number x number of moles

Question 3

What is the formula to calculate moles?

Answer 3

Moles = mass/Mr

Question 4

What is the formula to calc moles of a substance in solution?

Answer 4

Moles = Concentration x volume.

Make sure volume is in dm3
cm3 to dm3 is /1000

Question 5

Formula to calc conc in gdm-3?

Answer 5

Conc = Mass / Volume

Question 6

What is the empirical formula and how to work it put?

Answer 6

It is the simplest whole number ratio of elements in a compound

1. Write out the elements involved
2. Write the % as masses
3. Divide these by Mr to get the number of moles
4. Divide all the numbers by the smallest number of moles

Question 7

ionic equations

Answer 7

It shows the ions that are formed in solution and shows which particles are reacting. Only take the ions of the aqueous substances

H2SO4 + 2KOH —-> K2SO4 + 2H2O

2H+ + SO42- + 2K+ + 2OH- —> 2K+ + SO42- +2H2O
Cancel any ions that appear on both sides of the equation
2H+ + 2OH- —-> 2H2O

Question 8

Using equations to work out masses

Answer 8

Balanced equations can be used to calculate theoretical masses

How much CaO can be made when 34g of Ca is burnt completely in oxygen?
1. Write out the equation and balance it
2. Work out Mr of species involved. Write these as mass in grams
3. Divide the Ca side by 80 to find 1g then multiply by 34 to get 34g. Do the same for the CaO side

Question 9

Displacement reactions

Answer 9

These are reactions where a more reactive element takes the place of a less reactive element in a compound
F2(aq) + 2NaBr(aq) —-> 2NaF(aq) + Br2(aq)
F2(aq) + 2Br-9aq) —-2F-(aq) + Br2(aq)

Question 10

Acid base reactions

Answer 10

An acid and a base will form a salt and water. Carbonates will produce CO2 too.
Potassium nitrate
HNO3(aq) + KOH(aq) —-> KNO3(aq) + H2O(l)
H+(aq) + OH-(aq) —–> H2O(l)

Question 11

Precipitation reactions

Answer 11

When a solid is produced after 2 aqueous reactants react is called a precipitation reaction

BaCl2(aq) + K2SO4(aq) ——> 2KCl(aq) + BaSO4(s)
Ba2+(aq) + SO42-(aq) —–> BaSO4(s)

Question 12

The molar gas volume

Answer 12

Gases occupy the same volume of space under the same conditions

Number of moles = Volume(dm3) / Molar gas volume.

Molar Gas volume at standard room temp and pressure is 24dm3mol-1

Molar gas volume at standard temp and pressure is 22.4dm3mol-1

Question 13

The Ideal gas equation

Answer 13

pV=nRT

p = pressire in pascals
V = volume in m3
n = moles in mol
R = Gas constant (8.31)
K = Temperature in Kelvins

Question 14

Converting units.

Answer 14

M to dm is x10
dm to cm is x 10

m2 to dm2 is x 100
dm2 to cm2 is x 100

m3 to dm3 is x 1000
dm3 to cm3 is x1000

Question 15

What are hazards

Answer 15

Hazards are things that could cause harm to you or others around you

Question 16

What are risks?

Answer 16

Risks is the likelihood of the hazard causing harm to you or something else

Risks in experiments can range from:
Scalding from hot liquids
Burns from hot equipment
Burns from chemicals being used
Cuts from glass or metals
Harm or poisoning from gases and chemicals

Risk assessments should be carried out prior to any experiment being done. It needs to cover -
Identification of all hazards
The risk associated with each hazard
The likelihood of something going wrong
The seriousness of the risk
How to reduce the risk

Question 17

What are standard solutions?

Answer 17

They are made from solids with a known mass dissolved in water to a fixed volume in a volumetric flask. We can work out the concentration f this too

Question 18

How to make a standard solution?

Answer 18

1. Weigh out the amount of solid precisely using a balance and plastic/glass weighing boat
2. Transfer the solid from the weighing boat to a beaker. Wash any solid left behind into the beaker using deionised water.
3. Dissolve your solid fully using deionised water. Stir to ensure the solid is fully dissolved.
4. Transfer the solution to a volumetric flask. Use the funnel to avoid spillage and rinse the beaker and glass ro d into the flask to make sure most of the solution is transferred
5. Use more deionised water to fill of the graduation line. Be careful not to go above this line! Use a pipette to fill to the line when you get near.
6. Invert the flask a few times. Make sure you put a lid on it first! This ensures your solution is thoroughly mixed and ready to use

Question 19

Standard solution calculations?

Answer 19

1. Work out the number of moles of the solution you want to make
2. Use the number of moles from the previous step to calculate the mass

Question 20

How to carry out a titration?

Answer 20

1. Have acid or alkali in burette with a known concentration
2. Have an acid or alkali with an unknown concentration but a known volume in the conical flask. Add a few drops of indicator too
3. Add the chemical in the burette to the conical flask until the indicator changes colour. This is known as the endpoint. Add drop by drop near endpoint
4. Read how much chemical was added from the burette to neutralise the chemical in the conical flask. Read the bottom of the meniscus. Always read at eye level
5. Record your results to 2 d.p and repeat until you get 2 results that are concordant (within 0.10cm3 of eachother)

Question 21

What are the colours of the indicators in acid and alkali?

Answer 21

Phenolphthalein
in acid = colourless
in alkali = pink

Methyl orange
In acid = yellow
In alkali = red

Question 22

Titration calculation

Answer 22

1. Write out the equation and balance it
2. Calc the number of moles of HCl
3. Use the equation ot find out the molar ratio in order to work out the moles of KOH
4. Now calc concentration by using conc = moles / volume

Question 23

Errors and Uncertainty

Answer 23

Uncertainty is the degree of error your measurements have with a piece of equipment

The uncertainty is half the smallest increment the equipment can measure
In a burette there are marks every 0.1 cm3. You can read this burette to the nearest 0.05cm3 as any reading you take wouldn’t be more than 0.05cm3 out.
We can say the uncertainty of the reading will have a maximum error of +- 0.05cm3

If you are using more than one measurement with the same units you need to add the uncertainties

Question 24

How do you calculate percentage error?

Answer 24

% error = Uncertainty x by the number of measurements / Reading x 100

You can reduce % error by using more precise equipment. Also by using larger volumes of liquid

Question 25

What are the 2 types of errors?

Answer 25

Systematic errors - There are errors caused every time you repeat your experiment. Errors in equipment.

Random errors - These are errors caused by estimating readings on equipment. They vary with each repeat

Repeats and working out the mean will help to reduce the effects of random errors but not systematic errors.

Question 26

What is the % yield?

Answer 26

Actual yield / Theoretical yield x 100

The theoretical yield is the amount of a product produced assuming NO products are lost and ALL reactants react fully

Question 27

What is the atom economy?

Answer 27

It is how efficient a reaction is

Atom economy = The molecular mass of desired products/sum of molecular masses of all reactants x 100

It is always 100% for addition reactions as you are only forming 1 product

Question 28

What is the importance of the atom economy?

Answer 28

High atom economies produce less waste and so benefit the environment
High atom economies means that raw materials are used more efficiently. This is more sustainable
Companies will try to use reactions that tend towards the 100% atom economy.
Higher atom economy means less by-products so less time and money spent separating these from the desirable product