C2.3 Quantitative Chemistry and analysis

Question 1

What is the relative atomic mass of an element?

Answer 1

The comparison of the mass of atoms in an element with the carbon-12 isotope - it is the average value for isotopes of the element.

Question 2

What is the relative formula mass of a compound?

Answer 2

The sum of the relative atomic masses of each element in the compound.

Question 3

What is a mole?

Answer 3

The relative formula mass of a substance in grams

Question 4

What is the formula for calculating the number of moles?

Answer 4

number of moles = Mass in g/Mr (relative formula mass)

Question 5

Find the empirical formula of a compound formed from 2.4g of carbon and 0.8g of hydrogen.

Answer 5

Method:

C              H
2.4g        0.8g
/ 12          /1
= 0.2      = 0.8
x10           x10
= 2         = 8
= 1          = 4  (simplest form)

Formula = CH4

Question 6

Find the empirical formula of a compound formed from 19.2g of oxygen and 44.8g of iron.

Answer 6

Method:

0                 Fe
19.2g          44.8g
/ 16             /56
= 1.2          = 0.8
x 10           x 10
= 12          = 8
= 3           = 2
Formula = Fe2O3

Question 7

What mass of magnesium oxide is produced when 60g of magnesium is burned in the air?

Answer 7

Balanced Formula = 2Mg + O2 =====> 2MgO
Formula masses = (2x24) = 48 ======> (2x40) = 80
48g of Mg reacts to make 80g of MgO
1g of Mg, therefore, reacts to make 1.67g of MgO
60g of Mg, therefore, reacts to make 100g of MgO

Answer = 100g

Question 8

What mass of fluorine fully reacts with potassium to make 116g of potassium fluoride?

Answer 8

Balanced formula = F2 + 2K =====> 2KF
Formula masses = (19 x 2) = 38 ===> (2 x 58) = 116
So 38g of F reacts to form 116g of KF

Question 9

What is the relative mass of a proton?

Answer 9

1

Question 10

What is the relative mass of a neutron?

Answer 10

1

Question 11

What is the relative mass of an electron?

Answer 11

Very small

Question 12

Define the ‘yield’.

Answer 12

The amount of product obtained from a reaction

Question 13

What is the formula for percentage yield?

Answer 13

Percentage yield = actual yield/predicted yield x 100

Question 14

What is a ‘reversible reaction’?

Answer 14

A reaction where the products of a reaction can themselves react to produce the original reactants.

Question 15

Explain 3 reasons for a lower actual yield than predicted.

Answer 15

• The reaction may not go to completion because it is reversible so products will be reacting to make the original reactants
• Some of the product may be lost when separated from the reaction mixture (e.g filtration)
• Some of the reactants may react in ways different from the expected reaction

Question 16

Why is product yield important for sustainable development?

Answer 16

Using as little energy as possible to create the highest product yield possible means that resources are saved, a low yield means wasted chemicals.

Question 17

What is sustainable development?

Answer 17

Not using up resources quicker than they can be replaced so that there is enough for future generations.

Question 18

What is the purpose of paper chromatography?

Answer 18

To identify and separate artificial colours

Question 19

Explain the method of using paper chromatography to separate artificial colours.

Answer 19

• Extract the colour from the sample by placing it a small cup with a few drops of solvent.
• Put spots of the coloured solution on a pencil baseline on filter paper (pen might dissolve in the solvent)
• Roll up the sheet and put it in a beaker with some solvent, keeping the baseline above the level of solvent.
• The solvent will seep up the apper, taking the dyes with it. Different dyes form spots in different places.

Question 20

What is the purpose of gas chromatography?

Answer 20

To separate out a mixture of compounds and identify them

Question 21

Explain the method of using gas chromatography to identify and separate compounds.

Answer 21

• Different substances carried by a gas travel through a column packed with solid material at different speeds, so they become separated
• The number of peaks on the output chromatograph shows the number of compounds present
• The position of the peaks on the graph indicates retention time (the time taken for a substance to reach the detector). This helps identify the substances.

Question 22

Explain how adding a mass spectrometer to gas chromatography helps to identify compounds.

Answer 22

• It can identify substances quickly and accurately and can detect very small quantities.
• It can also give the relative molecular mass of each of the substances separated in the column, which is given by the molecular ion peak on a graph.

Question 23

What are the advantages of using instrumental methods in chemical analysis (3) ?

Answer 23

• They are very sensitive
• Very fast
• Very accurate