C2.3 Quantatice Chemistry and Analysis

Question 1

What does the atomic mass represent?

Answer 1

The protons and neutrons

Question 2

What does the atomic number represent?

Answer 2

The protons

Question 3

How do you find the number of neutrons in an element?

Answer 3

Atomic mass - Atomic number

Question 4

What is the relative mass of a proton?

Answer 4

1

Question 5

What is the relative mass of a neutron?

Answer 5

1

Question 6

What is the relative mass of an electron?

Answer 6

Very small 1/1836

Question 7

What are isotopes?

Answer 7

Different atomic forms of the same element which have the same number of protons but a different number of neutrons

Question 8

What’s changed from an element and its isotope?

Answer 8

It’s atomic mass, if the atomic number changes, it would be a different element

Question 9

What is the relative atomic mass (Ar)?

Answer 9

A way of comparing the masses of atoms of different elements - the Ar is the same as the atomic mass

Question 10

How is the relative atomic mass (Ar) measured?

Answer 10

How heavy atoms of an element are compared with the mass of an atom of carbon-12

Question 11

What is the relative atomic mass of an element which has more than one stable isotope?

Answer 11

An average value of all the different isotopes is taken in to count

Question 12

What is the relative formula mass (Mr)?

Answer 12

All the relative atomic masses of the atoms in a compound added together e.g. the Mr of Iron Oxide (Fe2O3) would be (2x56) + (3x16) = 160 (56=Ar of Fe) (16=Ar of O)

Question 13

What is a mole?

Answer 13

The Ar or Mr of a substance in grams is one mole of that substance e.g. Fe has an Ar of 56, so one mole is 56g

Question 14

What is the formula for finding the number of moles?

Answer 14

No. of moles = Mass in grams (of element /compound)
÷ Mr (of element/compound)

Question 15

What is the formula for finding the mass of a substance when you know the moles?

Answer 15

Mass = Moles x Mr

Question 16

What is percentage mass?

Answer 16

A way of saying what proportion of the mass of a compound is due to the atoms of a particular element

Question 17

What is the formula for finding the percentage yield of an element in a compound (%)?

Answer 17

Ar x No. of atoms (of that element) / Mr (of whole compound)

Question 18

What is the empirical formula?

Answer 18

The simplest possible whole number ratio of atoms of each element within that compound - sometimes the empirical formula is the same as the molecular of a compound e.g. MF of sodium hydroxide is NaOH, can’t be simplified so NaOH is the EF

Question 19

How do you find the empirical formula? (5 steps)

Answer 19

1. List all the elements in the compound
2. Underneath them, write their masses or percentages
3. Divide the mass/percentage by the Ar of that particular element
4. Take the values from step 3 and divide them by the smallest value from step 3 - this is the ratio of elements in the compound
5. If you get values that are not whole numbers, multiply all values until you get the lowest whole No.

Question 20

How would you work out the mass of a product from a certain mass of reactant? (This is a prediction of the yield)

Answer 20

1. Write out the balanced equation for the reaction
2. Find the Mr of the reactant and the product that you’re interested in
3. Now divide both Mr’s by the reactant’s Mr to get what 1g of reactant produces
4. Now multiply both sides by the mass given in the Q

Question 21

How would you work out the mass of a reactant from a certain mass of product?

Answer 21

1. Write out the balanced equation for the reaction
2. Find the Mr of the reactant and the product that you’re interested in
3. Now divide both Mr’s by the product’s Mr to get what 1g of reactant produces
4. Now multiply both sides by the mass given in the Q

Question 22

What is percentage yield?

Answer 22

A comparison between the amount of product you expect to get and the amount of product you actually get

Question 23

What is the formula for working out percentage yield (%)?

Answer 23

(Actual yield (grams) /Predicted yield (grams)) x 100

Question 24

Why are percentage yields never 100%? (3)

Answer 24

If the reaction is reversible, some of the products will react to make reactants, product is lost when it is separated from its reactants e.g. when you filter a liquid, you’ll always lose a bit of liquid and also, unexpected reactions may be happening

Question 25

How can you make an industrial process sustainable?

Answer 25

Use reactions with high percentage yields, use reactions that don't require much energy and use raw materials from renewable sources

Question 26

What is chemical analysis used for?

Answer 26

To identify the different additives in food

Question 27

What can paper chromatography be used for?

Answer 27

To find out what dyes are in a certain food colouring

Question 28

How do you carry out paper chromatography?

Answer 28

1. extract the colour from the food by mixing it with a few drop of solvent (e.g. water, ethanol, salt water etc.) 2. Put a spot of the solution on a pencil baseline on some filter paper (don't use pen because it might dissolve and mess everything up) 3. Put the sheet in a beaker with some solvent - keep the baseline above the solvent 4. The solvent seeps up the paper, taking the dyes with it - different dyes form spots in different places

Question 29

In paper chromatography, what is the filter paper called with the separated spots?

Answer 29

A chromatogram

Question 30

What does the chromatogram tell you in paper chromatography?

Answer 30

What different dyes colouring contain, if two different colourings travel the same distance, they're likely to be the same dye, it also tells you how many dyes a colouring contains

Question 31

What method allows you to identify different elements and compounds?

Answer 31

Instrumental methods

Question 32

What advantages are there to using instrumental methods over manual methods?

Answer 32

They are very sensitive and can detect tiny amounts of substances, they are very fast and can be automated (which saves time) and they are very accurate - machines don't make mistakes like humans do

Question 33

Give an example of an instrumental method

Answer 33

Gas chromatography

Question 34

How is gas chromatography carried out?

Answer 34

1. A gas is used to carry a mixture of substances through a column (tube) packed with a solid material 2. The substances travel through the tube at different speeds so they're separated 3. The time each substance takes to reach the detector is called the retention time. It can be used to help identify substances 4. The recorder draws a gas chromatogram

Question 35

What does the number of peaks on a GC chromatogram show?

Answer 35

The number of substances in the sample

Question 36

What can GC-MS be used for?

Answer 36

A mass spectrometer can identify substances leaving the column and can accurately detect very small quantities

Question 37

What does a mass spectrometer tell you?

Answer 37

It tells you the relative molecular mass of each substance, you can work out the Ar of the substances from the graph which is drawn - you just read of the x axis