Quantative Chemistry and Analysis

Question 1

What does the atomic number of an element tell us?

Answer 1

The number of protons in one atom of that element.

Question 2

What does the mad number of an element tell us?

Answer 2

The total number of protons and neutrons in one atom of that element.

Question 3

How can we find the number of neutrons in an atom?

Answer 3

Subtract the atomic number from the mass 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 term do we use to describe the relative mass of an electron?

Answer 6

Negligible

Question 7

What is an isotope?

Answer 7

Different atomic forms of the same element which have different numbers of neutrons.

Question 8

What is the symbol for relative atomic mass?

Answer 8

Ar

Question 9

What is the relative atomic mass in comparison to?

Answer 9

The mass of an atom of Carbon-12 which scientists have decided has a mass of exactly twelve.

Question 10

What two things are taken into account when finding the Ar of an element with different isotopes?

Answer 10

• The Ar of each isotope

* The abundance of each isotope

Question 11

What is the symbol for relative formula mass?

Answer 11

Mr

Question 12

What is relative formula mass?

Answer 12

The relative atomic masses of all the atoms in a compound added together.

Question 13

What is a mole?

Answer 13

The relative mass (Mr or Ar) of a substance in grams.

Question 14

What formula can be used to work out the number of moles in a substance?

Answer 14

Number of moles = Mass (g) / Mr (of substance)

Question 15

What is percentage mass?

Answer 15

A way of saying what proportion of the mass of a substance is due to a particular element.

Question 16

How can one find out percentage mass?

Answer 16

Percentage mass = ((Ar x number of atoms) / Mr (of compound))x100

Question 17

What is an empirical formula?

Answer 17

The simplest number of ratio of atoms of each element in a compound.

Question 18

What is the empirical formula of glucose (C6H12O6)?

Answer 18

CH2O

Question 19

How do you work out empirical formulae?

Answer 19

1) List all the elements in a compound.
2) Write their masses or percentages.
3) Divide each mass/percentage by the Ar of the element.
4) Divide all your answers by the smallest answer you get to find the ratio.
5) If you don’t get whole numbers, you need to multiply all the numbers so that you do.

Question 20

How do you work out the mass of a product?

Answer 20

1) Weite out a balanced equation for the reaction.
2) Find the Mr of the reactant and product you’re interested in.
3) Apply the “divide to get one, multiply to get all” rule:

• Divide both relative masses by the mass of the reactant.
• Multiply the answers by the amount of reactant given in the question.

Question 21

What is percentage yield?

Answer 21

The percentage yield is a comparison between the amount of product you can expect to get and how much you actually get.

Question 22

How do you work out percentage yield?

Answer 22

(Actual yield (g) / predicted yield (g)) x100

Question 23

What are three reasons why percentage yield is never 100%?

Answer 23

1) The reaction is reversible so the backwards reaction is taking place as well.
2) Product is lost when it’s separated from the reactants.
3) Unexpected reactions may be happening.

Question 24

What are three ways we can make chemical industrial processes more sustainable?

Answer 24

• Use reactions with high percentage yields.
• Use reactions that don’t require much energy.
• Use materials from renewable sources.

Question 25

How does one carry out paper chromatography?

Answer 25

1) Extract the colour from the food sample by placing it in a cup with solvent.
2) Put a spot of the coloured solution on a pencil baseline on some filter paper.
3) Put the sheet in a beaker with some solvent, being careful to keep the baseline above the solvent.
4) The solvent seeps up the paper, taking the dyes with it which form spots in different places in the paper.

Question 26

Why must the baseline in paper chromatography be drawn with pencil?

Answer 26

Because ink could dissolve into the solvent as well which would confuse the results.

Question 27

What is the filter paper you get at the end of paper chromatography called?

Answer 27

A chromatogram

Question 28

What can we learn from a chromatogram?

Answer 28

• What dyes a colouring contains.

* How many dyes a colouring contains.

Question 29

What is meant by the term instrumental method?

Answer 29

Methods that use machines

Question 30

What are tree advantages of using instrumental methods?

Answer 30

• They are very sensitive.
• They are very fast and can be automated.
• They are very accurate.

Question 31

What is an example of an instrumental method?

Answer 31

Gas chromatography

Question 32

What does gas chromatography do?

Answer 32

Separate out a mixture of compounds to help us identify the substances present.

Question 33

How does gas chromatography work?

Answer 33

• A gas carries the mixture through a column packed with solid material.
• The substances travel at different speeds through the column so are separated out.
• The time it takes for each substance to reach the detector at the other end of the column is recorded - this is called the retention time.
• The detector sends information to the recorder which draws a gas chromatogram.

Question 34

How can we tell how many substances were present in a mixture used in gas chromatography?

Answer 34

Each peak on the chromatogram represents a different substance so you just count the peaks.

Question 35

What is GC-MS?

Answer 35

Gas Chromatography- Mass Spectrometry is a process that is used to identify substances in a mixture in very small quantities.

Question 36

How does GC-MS help us identify substances?

Answer 36

It tells us the relative molecular mass of each substance.