3 How much?

Question 1

What is the relative mass of the three main sub particles?

Answer 1

Proton - 1
Neutron - 1
Electron - very small negligible mass

Question 2

What is the mass number?

Answer 2

The total number of protons and neutrons in an atom

Question 3

What is the atomic number?

Answer 3

The number of protons in an atom

Question 4

How can you work out the total number of neutrons in an atom?

Answer 4

number of neutrons = mass number - atomic number

Question 5

What is an isotope?

Answer 5

Atoms of the same element with different numbers of neutrons (protons are always the same)

Question 6

What is an example of an element with different isotopes?

Answer 6

Carbon has two common ones: 6 neutrons and 8 neutrons
Hydrogen has three: 1 neutron, 2 neutrons and 3 neutrons
Chlorine has two: 35 neutrons and 37 neutrons (makes up 75% and 25% accordingly so it’s averaged like this)

Question 7

What can extra neutrons in a nucleus cause?

Answer 7

Can make the nucleus unstable so it is radioactive however not all isotopes are radioactive

Question 8

How do properties change in different isotopes of the same element?

Answer 8

May have different physical properties -e.g.density, mass radioactivity- but never any change in chemical properties because it is still the same element

Question 9

Why do the chemical properties of isotopes never change?

Answer 9

Their reactions depend on their electronic structure- as they have the same number of electrons, the electronic structure will be the same for all isotopes of that element

Question 10

What is relative atomic mass?

Answer 10

The mass of an element compared to the carbon 12 atom (because it is stable with an equal number of protons and neutrons)
known as Ar
This tells us that carbon is 12 times as heavy as hydrogen and helium as 4 times heavier than hydrogen

Question 11

What does relative atomic mass also take into account?

Answer 11

The isotopes of certain elements

Question 12

What is the relative atomic mass of an element in grams called?

Answer 12

1 mole

Mr

Question 13

How many moles is: 12g of carbon, 48g of carbon and 36g of water?

Answer 13

1- 1 mole of carbon
2- 4 moles of carbon (12 x 4= 48)
3- 2 moles of water ((1 x 2)+ 16 = 18(1 mole of water) )

Question 14

How do you work out the relative atomic mass of elements with different isotopes? show example

Answer 14

chlorine 35= 75%
chlorine 37= 25%

((35 x 75)+(37 x 25))
—————————– = 35.5 amu
100

Question 15

Why do atoms have different isotopes?

Answer 15

Because there are different ways of stabilising the protons

Question 16

What is relative formula mass?

Answer 16

The relative formula mass (Ar or Mr) of a substance in grams is known as one mole of that substance
The sum of masses of all the atoms they contain (can be known as RMM- relative molecular mass)

Question 17

How do you work out the number of moles in a substance?

Answer 17

Number of moles = mass in g of element or compound
—————————————————-
relative formula mass of element or compound

Question 18

How do you work out the percentage mass of an element in a compound?

Answer 18

Total mass of certain element
——————————————— x 100
Total molecular mass

Question 19

What is the empirical formula?

Answer 19

The ratio of atoms in a molecule

Question 20

Go through a worked example of the steps to work out the empirical formula of a compound?
(compound made by reacting 1.92g of magnesium with 5.68g of chlorine)

Answer 20

e.g. compound made by reacting 1.92g of magnesium with 5.68g of chlorine
Magnesium Chlorine
mass (g)= 1.92 5.68
molar mass (g)= 24 35.5

(Mass divided by molar mass)
no. of moles= 1.92/24=0.08 5.68/35.5=0.16

(Amount divided by smallest amount)
= 0.08/0.08=1 0.16/0.08=2

ratio= 1 2

FORMULA= MgCl2

Question 21

Work out the relationship of mass, molar mass and number of moles into an equation triangle…

Answer 21

Mass

              - ---------------------------------
           no. of moles   I   molar mass

Question 22

What are the three important steps in calculating masses in reactions?

Answer 22

1- write out and balance the equation
2- work out the relative formulas mass of the necessary parts
3- Divide to get 1g of the substance and times to the specified amount

Question 23

What is ‘the yield’?

Answer 23

Describes how much product is made from a chemical reaction

Question 24

How do you calculate percentage yield?

Answer 24

amount of product produced (actual)
———————————————————————- x 100
maximum amount of product possible (predicted)

Question 25

Why do very few reactions have a yield of 100%? What reasons are there for losing product?

Answer 25

• The reaction may be reversible (so as products form they react to form the reactants again)
• Some reactants may react to give unexpected products (waste/by-products)
• Some of the product may be lost in handling or left behind in the apparatus
• The reactants may not be completely pure
• Some chemical reactions produce more than one product which may make it difficult to separate the product we want from the reaction mixture
• The reaction may be incomplete

Question 26

What is atom economy?

Answer 26

Making as much useful product as possible is important for companies finances and the environment too. The amount of starting materials that end up as useful products is called atom economy- the aim is to achieve maximum atom economy for sustainability

Question 27

How do you calculate percentage atom economy?

Answer 27

mass of useful products
———————————— x100
mass of all products

Question 28

What is a reversible reaction?

Answer 28

A reversible reaction is where the products of the initial reaction themselves react, producing a reverse raction

Question 29

Describe the symbol for a reversible reaction…

Answer 29

Two half arrows pointing either way, lying horizontal and parallel to eachother

Question 30

What happens during the reversible reaction of heating ammonium chloride?

Answer 30

• breaks down on heating: thermal decomposition
  ammonium chloride (heat)–> ammonia + hydrogen chloride
• two gases rise up the test tube and when they cool they react with each other re-forming ammonium chloride again
• a white solid forms on the inside of the glass
  ammonia + hydrogen chloride = ammonium chloride

Question 31

What are food additives?

Answer 31

Any substances, natural or synthetic, that is added to food to extend its shelf life, improve its taste or improve its appearance- we can test for food additives using basic analytical techniques

Question 32

How does chromatography work? What type of method is it?

Answer 32

We can separate different colours in foods as some colours dissolve well and travel a long way up the paper whereas others don’t and come further down the paper- we can then see the colours separated out
Paper chromatography is a non-instrumental method for analysing substances

Question 33

Where are instrumental methods used?

Answer 33

They are used in industry and are also important in fighting pollution as the environment can be carefully monitored with these machines- they are also used in healthcare

Question 34

What are the advantages and disadvantages of instrumental methods?

Answer 34

Advantages:

• they are highly accurate and sensitive
• they are quicker
• they enable very small samples to be analysed
• reduces human error

Disadvantages:

• very expensive usually
• takes special training to be used
• gives results that can often be interpreted only by comparison with data from known substances
• requires maintenance
• can malfunction/ humans can still wrongly interpret data

Question 35

How does gas chromatography work?

Answer 35

The mobile phase (when different compounds pass through at different speeds depending on how ‘soluble’ they are in the stationary phase) is an inert gas e.g. helium or nitrogen- known as carrier gas- the stationary phase is a liquid covering the surface of a solid material, packed into a long tube called the column
The substances travel at different speeds, separating and the detector records the arrival of each chemical from the column

Question 36

In gas chromatography what do the number of peaks indicate?

Answer 36

the number of compounds present

Question 37

In gas chromatography what does the size of the peak indicate?

Answer 37

the relative amount of each chemical present

Question 38

In gas chromatography what does the position of the peaks indicate?

Answer 38

the retention time

Question 39

What is the retention time?

Answer 39

This is the time taken for each component to pass through the stationary phase
It can be used to identify compounds by comparison with retention times of known compounds

Question 40

What is mass spectrometry and how does it work?

Answer 40

The output of the gas chromatography column can be linked to the mass spectrometer which is able to identify the substances leaving the column
It can identify substances quickly and accurately in very small quantities

Question 41

Other than identifying substances, what else can the mass spectrometer do?

Answer 41

It can also give the relative molecular mass of each substance
The molecular mass is given by the molecular ion peak- this is the heaviest ion, corresponding to mass of sample molecule with one electron removed (it is always on the right hand side of the mass spectrum)