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What is the atomic number?

Number of protons (= Number of electrons)


What is the mass number?

Total number of protons + neutrons in nucleus.


Why do we only count protons and neutrons to calculate the mass number of an atom?

Because the mass of an electron is very small compared with a proton and neutron, so the mass of an atom is made up almost entirely of its protons and neutrons.


What is an isotope?

Atoms of the same element but with different numbers of neutrons are called isotopes.


How many protons, neutrons and electrons are then in an atom of 19= mass number, 9= atomic number?

9 protons.

9 electrons.

19-9= 10 neutrons.


Why do we use relative atomic masses?

Because atoms are too small to weigh.


What is relative atomic mass also written as?



What is a mole?

One mole of any substance is its relative formula mass in grams.


What is relative atomic mass?

The relative atomic mass of an element is an average value for the isotopes of an element.


How do we work out the relative formula mass (Mr)?

We work out the relative formula mass of a compound by adding up the relative atomic masses of the elements in it.


Calculate the relative formula mass (Mr) of sodium sulfate, Na2SO4.

(Ar: Na= 23, S=32, O= 16)

(2 x 23)+32 +(4 x 16)= 142


What is the mass of one mole of magnesium carbonate, MgCO3?

(Ar: Mg= 24, C= 12, O= 16)

24+ 12+ (16 x 3)= 84g


How do you calculate percentage composition of an element in a compound?

Percentage= (Relative atomic mass x no. of atoms)/ relative formula mass x 100%


What is the percentage of carbon in methane, CH4?

(Ar of C= 12, Ar of H= 1)

Mr= 12+ (4 x 1)= 16

Ar of C= 12

(12/16)x 100%= 75%


What is the empirical formula?

The empirical formula is the simplest ratio of atoms or ions in a compound.

The empirical formula is the formula used for ionic compounds, but for covalent compounds it is not always the same as the molecular formula.

For example: Molecular formula of ethane= C2H6 but empirical formula of ethane= CH3.


How can we calculate the empirical formula?

1. Write down mass of each element.

2. Divide mass by relative atomic mass of element.

3. Divide number by smallest number get ratio of elements.

4. These numbers give the empirical formula.


A compound has 24g of carbon and 64g of oxygen.

What is the empirical formula?

Mass of carbon= 24.

Mass of oxygen= 64. 

Mass/Ar= 24/12=2.

Mass/ Ar= 64/16=4. 

Divide by smaller no.= 2/2=1.

Divide by smaller no.= 4/2=2. 


Empirical formula= CO2.


What is the empirical formula of the hydrocarbon that contains 80% carbon?

Mass in 100g= 80g carbon, 20g hydrogen.

Mass/Ar= 80/12= 6.67 carbon, 20/1= 20 hydrogen.

Divide by smallest= 6.67/6.67=1 carbon, 20/6.67= 3 hydrogen.

Empirical formula= CH3


There are two types of chromatography.

What are they?

1. Paper.

2. Gas.


What is paper chromatography?

Paper chromatography is a technique used when you want to seperate components of a mixture (water-soluble).

It can be used to analyse artificial colours in food.


How does paper chromatography work?

1. A spot of colour is put onto paper.

2. A solvent is allowed to move through the paper.

3. The colours move different distances depending on their solubility.


What is gas chromatography?

Gas chromatography is a technique for separating components of a vapourised sample.


How does gas chromatography work?

1. Mixture carried by inert 'carrier' 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. Amount of substance recorded against times.

This shows the number of compounds in the mixture and their retention times.


What can be done with the retention times in gas chromatography?

They can be compared with results for known compounds to help identify the compounds in the mixture.


The output from the gas chromatography can be linked to a mass spectrometer.

What does the mass spectrometer do?

This can be used to find the relative molecular mass of a compound from its molecular ion peak.


What is the relative mass of protons and neutrons?



Give some advantages and disadvantages to using gas chromatography.

A- Highly accurate and sensitive.

A- Relatively quick results.

A- Can analyse very small samples.

D- Expensive.

D- Specialist training required.

D- Results only interpreted by comparison of known results, or mass spectrometer.


A sample of copper nitride contains 3.81g of copper and 0.28g of nitrogen.

Calculate the empirical formula.

(Relative atomic masses: N= 14, Cu= 63.5)

3.81/ 63.5= 0.06.

0.28/14= 0.02.



What information does the molecular ion peak give about the molecule?

Relative molecular mass.


The relative atomic mass of an element compares the mass of an atom of an element with the mass of an atom of...