Atomic structure, analysis and quantitative chemistry

Question 1

What is the mass number of an atom?

Answer 1

The total number of protons and neutrons in an atom.

Question 2

What are isotopes?

Answer 2

Atoms of the same element that have different numbers of neutrons but the same number of protons.

Question 3

How is the relative atomic mass calculated?

Answer 3

We use an atom of carbon-12 and compare the masses of all other atoms with this. The relative atomic mass of an element is an average value for the isotopes of the element. When rounded to a whole number it is often the same as the mass number of the main isotope of the element.

Question 4

What is a mole?

Answer 4

The relative atomic mass of a substance, in grams.

Question 5

Formula for working out the percentage of an element in a compound.
When can this be useful?

Answer 5

relative atomic mass of element ÷ relative formula mass of compound x 100
When deciding if a compound is suitable for a particular purpose or to identify a compound.

Question 6

What is the empirical formula of a compound?

What is different about the empirical formula of ionic and covalent compounds?

Answer 6

The simplest whole number ratio of atoms or ions in a compound.
For covalent compunds it is not always the same as the molecular formula.

Question 7

How can the empirical formula of a compound be calculated from its percentage composition? (2)

Answer 7

1) Divide the mass of each element in 100g of the compound by its Ar, to give the ratio of atoms.
2) Then convert this to the simplest whole number ratio, i.e. divide by the smallest.

Question 8

For what reasons may the yield of a chemical reaction be less than 100%? (3)

Answer 8

1) Reaction may not go to completion.
2) Some product may be lost when it is collected from the apparatus.
3) Some product may be lost when it is separated.

Question 9

What is the formula for percentage yield?

Answer 9

Percentage yield = amount of product collected ÷ maximum amount of product possible x 100%

Question 10

How is the maximum amount of product possible calculated?

Answer 10

Using balanced symbol equation.

Question 11

Why are reactions with high yields used in industry?

What else should chemical processes do and why?

Answer 11

To conserve resources and reduce waste.

Waste as little energy as possible, this helps to reduce pollution and make production more sustainable.

Question 12

What are food additives?

Answer 12

Substances added to food to improve taste, appearance or shelf life.

Question 13

How and why do food scientists analyse foods to identify additives?

Answer 13

Paper chromatography, to ensure only safe, permitted additives have been used.

Question 14

How might compounds in a mixture be separated and then how can they be identified?

Answer 14

Gas chromatography and then mass spectrometry.

Question 15

Why is argon often used as a carrier gas in gas chromatography?

Answer 15

It is inert and cheaper than other inert gases.

Question 16

What is meant by the retention time?

Answer 16

The time taken for a component in a mixture to pass through the column in gas chromatography.

Question 17

Why are modern instrumental methods of analysis useful?

What are the drawbacks?

Answer 17

They are accurate, sensitive and rapid and are particularly useful when the amount of a sample is very small.
The equipment is usually very expensive and requires training to operate it.

Question 18

What information can a mass spectrometer give about a substance?
For an individual compound to what does the peak with the largest mass correspond?
What is the peak called and where can it be found on a mass spectrum?

Answer 18

The Mr of the substance.
It corresponds to an ion with one electron removed, this peak is called the molecular ion peak and is furthest to the right on a mass spectrum.

Question 19

How is the relative formula mass (Mr) of a substance calculated?

Answer 19

By adding up the relative atomic masses of the atoms in its formula.

Question 20

Calculating masses from chemical reactions.

Answer 20

Practice Page 44 revision guide.

Question 21

What is the difference between the yield and percentage yield of a chemical reaction?

Answer 21

The yield of a chemical process is how much product you actually make.
The percentage yield compares the amount you actually make with the maximum amount that could be made, calculated as a percentage.

Question 22

What is a reversible reaction?

Example of a reversible reaction?

Answer 22

A reaction where the products react to make the reactants, i.e the reaction can go in both directions.

Ammonium chloride ammonia + hydrogen chloride

When heated, ammonium chloride decomposes to produce ammonia and hydrogen chloride. When cooled, ammonia and hydrogen react to produce ammonium chloride.

Question 23

Detail the main steps in the process of paper chromatography when used to analyse the artificial colours in food. (3)

Answer 23

1) A spot of colour is put onto paper and a solvent is allowed to move through the paper.
2) The colours move different distances depending on their solubility.
3) The distances moved can be compared to that of known colourings.

Question 24

What types of substance may require modern instrumental methods to be annalysed?

Answer 24

Samples for analysis are often mixtures that need to be separated so that the compunds can be identified.

Question 25

Explain the process of gas chromatography. (5)

Answer 25

1) In gas chromatography the mixture is carried by a gas through a long column packed with particles of a solid.
2) The individual compounds travel at different speeds through the column and come out at different times.
3) The amount of substance leaving the column at different times is recorded against time and shows the number of compounds in the mixture and their retention times.
4) The retention times can be compared with the results of known substances to help identify the compounds in the mixture.
5) The output from a gas chromatography column can be linked directly to a mass spectrometer, which gives further data that a computer can use to help identify the individual compounds.