Quantatative Chemistry and Analysis Of Substances

Question 1

What are relative masses of protons, neutrons and electrons?

Answer 1

Proton:1
Neutron:1
Electron: 1/2000 or very small

Question 2

What is the mass number of an atom?

Answer 2

The total number of protons and neutrons it contains

Question 3

What are isotopes?

Answer 3

Isotopes are the same element with the SAME amount of PROTONS
But DIFFERENT amount of neutrons, so there mass number will be different.

Question 4

How do we calculate the number of neutrons of an atom?

Answer 4

Mass number( on top) - atomic number(bottom)

Question 5

What is the relative atomic mass?( top number on periodic table).

Answer 5

The Relative Atomic Mass of an element is the mass of an “average atom” of that element (taking into account its different isotopes and their relative proportions in that average) compared with the mass of an atom of carbon-12.

Question 6

What is the relative formula mass(Mr)

Answer 6

It is the sum of the relative atomic mass of an element. It is measured in grams and is known as one mole of that substance

Question 7

How do you work out the amount of moles a substance?

Answer 7

The mass given/ formula mass

Question 8

Describe paper chromatograpy

Answer 8

Paper chromatography is used to analyse coloured substances, such as artificial colours in food additives.
Extract the colour from a substance by placing it in a cup with a small amount of solvent.
We place an extracted drop of the substance on the baseline (drawn in pencil)of the paper. It is then placed in a beaker containing water or ethanol(solvents).

The solvent seeps up the paper taking the dyes with it. Different dyes form spots in different places.

Question 9

How do you work out the amount of moles a substance?

Answer 9

The mass given/ formula mass

Question 10

Why does paper chromatography work?

Answer 10

Because some compounds in a mixture dissolve better than others in particular solvents.

Once compounds have been separated, they can be identified by comparing the chromatogram with others obtained from known substances.

Control when comparing :same solvent and temperature

Question 11

How do we know if substances are the same in paper chromatography?

Answer 11

Same colour, same distance travelled up the paper and same Rf value( ratio between distance travelled by dissolved substance and solvent)

Question 12

What are the advantages of instrumental methods?

Answer 12

Highly accurate and sensitive

They are quicker

They enable very small samples to be analysed

Question 13

What are the disadvantages of instrumental methods?

Answer 13

They are usually very expensive

It takes special training to use

It gives results that can often only be interpreted by comparison with a known source.

Question 14

What are reversible reactions?

Answer 14

Where products can react together to make original reactants again

For e.g ammonium chloride breaks down on heating and forms ammonia and hydrogen chloride

However when they cool down they react with each other and reform ammonium chloride. A white solid is reformed.

Question 15

Describe gas chromatography

Answer 15

First of all the sample mixture is vaporised
A carrier gas, usually nitrogen moves the vapour through a coiled column
The compounds in the sample all have different attractions to the material in the column

The compounds with strong attractions to the material in the column will take longer to get through the column(long retention times)

The compounds with weak attractions to the material in the column leave it first( they have short retention times).

We can identify unknown substances in a sample by comparing the chromatograph with the results for known substances

Question 16

Why is mass spectrometry useful( reason 1)?

Answer 16

To ensure that we identify the unknown substances, gas chromatography apparatus can be attached to a mass spectrometer. This identifies substances very quickly and accurately and can detect very small quantities in a sample.

Question 17

Why is mass spectrometry useful?( reason 2)?

Answer 17

If provides an accurate way of measuring the relative formula mass of a compound.

The peak with the largest mass corresponds with an ion with just one electron removed.

The molecular ion peak of a substance is always found as the last peak on the right as you look at a mass spectrum.
The molecular ion peak gives us the relative formula mass of a substance.

Question 18

How do you work out percentage composition?

Answer 18

Mr of element/Mr of compound X100

Question 19

What is conservation of mass?

Answer 19

Mass is never lost or gained in a chemical reaction. Total mass of products at the end of the reaction is equal to the total mass of reactants at the beginning.

Question 20

How do we calculate percentage yield?

Answer 20

Percentage yield= Amount of product produced/ maximum product produced * 100%

The answer is given as a percentage.

Question 21

Why do very few reactions have a 100% yield?

Answer 21

The reaction may be reversible
Some may be lost to surroundings or left on apparatus
Some reactants may not be completely pure
Some reactants may give unexpected products
Some chemical reactions produce more than one product, may be difficult to separate the product we want from the reaction mixture.

Question 22

How do we calculate the empirical formula?

Answer 22

You are normally giving their mass, right this down.

Then look up the Ar( relative atomic mass) values for each element

Then divide the masses or sometimes percentages by the Ar of each element

Once that is done, you should get a ratio, the ratio equals the number of atoms there are( you might need to cancel down, or multiply to get a whole number ratio.

Question 23

How do we calculating reacting masses?

Answer 23

Mr of the compound we are asked to make/ Mr of the element we are given * mass of element given