C2- How Much?

Question 1

What is the atomic number?

Answer 1

Number of protons (= Number of electrons)

Question 2

What is the mass number?

Answer 2

Total number of protons + neutrons in nucleus.

Question 3

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

Answer 3

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.

Question 4

What is an isotope?

Answer 4

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

Question 5

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

Answer 5

9 protons.

9 electrons.

19-9= 10 neutrons.

Question 6

Why do we use relative atomic masses?

Answer 6

Because atoms are too small to weigh.

Question 7

What is relative atomic mass also written as?

Answer 7

A_r.

Question 8

What is a mole?

Answer 8

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

Question 9

What is relative atomic mass?

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

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

Answer 11

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

Question 12

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

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

Answer 12

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

Question 13

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

Answer 13

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

Question 14

What is the percentage of carbon in methane, CH₄?

(A_r of C= 12, A_r of H= 1)

Answer 14

M_r= 12+ (4 x 1)= 16

A_r of C= 12

(12/16)x 100%= 75%

Question 15

What is the empirical formula?

Answer 15

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= CH_3.

Question 16

How can we calculate the empirical formula?

Answer 16

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.

Question 17

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

What is the empirical formula?

Answer 17

Mass of carbon= 24.

Mass of oxygen= 64.

Mass/A_r= 24/12=2.

Mass/ A_r= 64/16=4.

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

Divide by smaller no.= 4/2=2.

Ratio=1:2.

Empirical formula= CO₂.

Question 18

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

Answer 18

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

Mass/A_r= 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

Question 19

There are two types of chromatography.

What are they?

Answer 19

1. Paper.
2. Gas.

Question 20

What is paper chromatography?

Answer 20

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.

Question 21

How does paper chromatography work?

Answer 21

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.

Question 22

What is gas chromatography?

Answer 22

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

Question 23

How does gas chromatography work?

Answer 23

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.

Question 24

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

Answer 24

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

Question 25

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

What does the mass spectrometer do?

Answer 25

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

Question 26

What is the relative mass of protons and neutrons?

Answer 26

1.

Question 27

Give some advantages and disadvantages to using gas chromatography.

Answer 27

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.

Question 28

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)

Answer 28

3. 81/ 63.5= 0.06.
4. 28/14= 0.02.
   3: 1➡️Cu₃N.

Question 29

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

Answer 29

Relative molecular mass.

Question 30

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

Answer 30

Carbon-12.

Question 31

The start line in paper chromatography should be drawn in pencil, not ink.

Suggest why.

Answer 31

The ink would dissolve in the solvent.