C2- atomic structure, analysis and quantitative chemistry

Question 1

What will all atoms of a particular element have the same?

Answer 1

The atoms of a particular element will all have the same number of protons. Their atomic number will be the same.

Question 2

What are isotopes?

Answer 2

Atoms of the same element with different numbers of neutrons are called isotopes. The different isotopes of an element have identical chemical properties. However, some isotopes are radioactive.

Question 3

What is the symbol for relative formula mass?

Answer 3

Mr (small r)

Question 4

What is the symbol for relative atomic mass?

Answer 4

Ar (small r)

Question 5

How do you work out the relative formula mass?

Answer 5

To find the relative formula mass of a substance, you just add together the relative atomic mass values for all the atoms in its formula.

Question 6

What is one mole of a substance?

Answer 6

The relative formula mass of a substance - shown in grams - is called one mole of that substance. For example, the Mr of carbon monoxide (CO) is 28. This means that one mole of carbon monoxide has a mass of 28 g. You should be able to see that:

14 g of carbon monoxide contains 14 ÷ 28 = 0.5 moles
56 g of carbon monoxide contains 56 ÷ 28 = 2 moles

Question 7

What is the relative atomic mass?

Answer 7

The relative atomic mass of an element (Ar) is an average value for the isotopes of the element. For example, the Ar for chlorine is 35.5 because it contains two different isotopes.

Question 8

What do you work out the relative atomic mass (chlorine is provided as an example)?

Answer 8

75 per cent of chlorine atoms are 35Cl and 25 per cent of chlorine atoms are 37Cl.

This means that in 100 chlorine atoms, 75 will be 35Cl and 25 will be 37Cl.

The total Ar for these chlorine atoms will be (75 × 35) + (25 × 37) = 2625 + 925 = 3550.

So the average Ar for chlorine is 3550 ÷ 100 = 35.5.

Question 9

What is the standard atom?

Answer 9

The mass of the 12C (upper 12) isotope is the ‘standard atom’ that the masses of other atoms are compared to. The Ar of 12C is defined as 12. This is also Avogadro’s number, which is the amount of amounts in 12 grams of carbon-12, which is 6.02x10^23

Question 10

What is chromatography used for?

Answer 10

Chromatography is used to separate mixtures of substances into their components.

Question 11

What is paper chromotopraghy used for?

Answer 11

Paper chromatography is used to analyse coloured substances

Question 12

What is the method of paper chromotography?

Answer 12

Samples of each substances are spotted onto a pencil line drawn on a sheet of chromotography paper. The paper is suspended in a container with a shallow layer of a suitable solvent or mixture of solvents in it. It is important that the solvent level is below the line with the spots on it. The container should be covered. As the solvent slowly travels up the paper, the different components of the ink mixtures travel at different rates and the mixtures are separated into different coloured spots.

Question 13

Why is it important that the container is covered?

Answer 13

The reason for covering the container is to make sure that the atmosphere in the beaker is saturated with solvent vapour. Saturating the atmosphere in the beaker with vapour stops the solvent from evaporating as it rises up the paper.

Question 14

Why does paper chromotography work?

Answer 14

Paper chromatography works because some of the coloured substances are better at dissolving in the liquid than they are at bonding with the paper, so they travel further up the paper.

Question 15

What are instrumental methods of analysis?

Answer 15

Instrumental methods of analysis rely on machines. There are several different types of instrumental analysis.

Question 16

What are the advantages of instrumental methods of analysis?

Answer 16

Some are suitable for detecting and identifying elements, while others are better suited to compounds. In general, instrumental methods of analysis are:

Fast
Accurate (they reliably identify elements and compounds)
Sensitive (they can detect very small amounts of a substance in a small amount of sample)

Question 17

What does Gas chromatography do?

Answer 17

Gas chromatography allows a mixture of compounds to be separated.

Question 18

What does a gas chromatography machine consist of?

Answer 18

The GC machine consists of a long glass tube packed with a powdered solid material, which is fitted into an oven. The tube is called the column, even though it is usually wound into a coil so that it fits into the oven.

Question 19

How does gas chromatography work?

Answer 19

The sample is dissolved in a solvent, then injected into one end of the column.
An unreactive gas - usually nitrogen - carries the sample through the column.
Different substances in the sample travel through the column at different speeds and so become separated from each other.
The separated substances leave the column one after the other. As they leave, they are detected by a detector.

Question 20

What is the retention time?

Answer 20

The time taken for a substance to travel through the column is called its retention time.

Question 21

What does the graph produced by the detector show?

Answer 21

A detector produces a graph where each substance is represented by a peak:

The number of peaks shows the number of compounds present in the sample
The position of each peak shows the retention time for each compound

Question 22

What does the mass spectrometer do?

Answer 22

The mass spectrometer can be used to identify substances quickly and accurately, and in very small amounts. It can also provide the relative formula mass of the substances separated by gas chromatography. If a GC machine is linked to a mass spectrometer, this combined GC-MS method allows the separated substances leaving the column of the GC machine to be detected.

Question 23

What is the molecular ion peak?

Answer 23

The peak furthest to the right in a mass spectrum is called the ‘molecular ion peak’. Its relative mass is the relative formula mass of the substance being analysed.

Question 24

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

Answer 24

Find the relative atomic mass of the element you want ( if there is more than one atom of the element in the compound, you need to multiply your answer by the number of atoms.) Then divide this number by the relative formula mass of the compound, and then times by 100 to get the percentage.

Question 25

What does the conservation of mass mean?

Answer 25

Mass is never lost or gained in chemical reactions. We say that mass is conserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning.

Question 26

What does the conservation of mass help you work out?

Answer 26

This fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. It also helps you calculate the theoretical mass of product expected in a chemical reaction

Question 27

Why is it not always possible in practice to get the entire calculated amount of product?

Answer 27

Reversible reactions may not go to completion
Some product may be lost when it is removed from the reaction mixture
Some of the reactants may react in an unexpected way

Question 28

What is the theoretical yield?

Answer 28

The theoretical yield is the maximum theoretical mass of product in a reaction (calculated using the idea of conservation of mass)

Question 29

What is the actual yield?

Answer 29

The actual yield is the mass of product you get when you actually do the reaction

Question 30

What is the percentage yield?

Answer 30

The percentage yield is the ratio of actual mass of products obtained compared with the maximum theoretical mass.

Question 31

How do you calculate the percentage yield?

Answer 31

percentage yield = (actual mass of product) ÷ (theoretical mass of product) × 100

Question 32

How do we show a reversible reaction?

Answer 32

When writing chemical equations for reversible reactions, we do not use the usual one-way arrow. Instead, we use two arrows, each with just half an arrowhead - the top one pointing right, and the bottom one pointing left.

Question 33

What does ammonium chloride break down to form?

Answer 33

ammonium chloride (a white solid) can break down to form ammonia and hydrogen chloride. It also shows that ammonia and hydrogen chloride (colourless gases) can react to form ammonium chloride again.

Question 34

What is the reversible reaction involving white anhydrous copper(II) sulfate and blue hydrated copper(II) sulfate,

Answer 34

anhydrous copper(II) sulfate + water (double headed arrow) hydrated copper(II) sulfate

Question 35

What is the test for the presence of water?

Answer 35

The reaction between anhydrous copper(II) sulfate and water is used as a test for water. The white solid turns blue in the presence of water.

Question 36

How would you work out how much of a reactant/product you can make? For example: If we have 50g of CaCO3, how much CaO can we make?

Answer 36

If you have a balanced equation for a reaction, you can calculate the masses of reactants and products.
First, work out the Mr values for the two compounds:

Mr of CaCO3 is 40 + 12 + 16 + 16 + 16 = 100

Mr of CaO is 40 + 16 = 56

This means that 100 g of CaCO3 would yield 56 g of CaO in this reaction. In the question we are told we have only half of that amount of CaCO3, 50 g. So we will get half the amount of CaO, 28 g.

So the mass of CaO we can make = 28 g

Question 37

How do you calculate the formula of a compound?

Answer 37

First, you find the masses of elements (usually given to you in the question.) And then you divide it by the relative atomic mass of each element. Then, divide each number by the smaller number produced in order to find the ratio. (If one of the numbers ends in 0.5, multiply all the numbers by 2 - this is because you cannot have half-atoms in a compound.)