Quantative Chemistry

Question 1

What is the definition of relative atomic mass?

Answer 1

The average mass of all the isotopes of that element.

Question 2

What is the symbol for relative atomic mass?

Answer 2

Ar

Question 3

On a periodic table, is the relative atomic mass the larger or smaller of the 2 numbers for each element?

Answer 3

The larger number

Question 4

How do you find the relative formula mass of a compound?

Answer 4

• Write down how many atoms of each element there are, and their relative atomic masses.
• Multiply the number of atoms of each element by their relative atomic mass.
• Add all the values together.

Question 5

What is the symbol for relative formula mass?

Answer 5

Mr (sometimes you may see it as ‘RFM’).

Question 6

What is the formula that links moles, mass and Mr?

Answer 6

mass = moles x Mr

Question 7

mass = moles x Mr

Rearrange the above formula to get moles by itself.

Answer 7

moles = mass ÷ Mr

Question 8

What is a mole?

Answer 8

A mole is a unit of measure (like a mile, hour, or litre).

1 mole of a substance is the amount of that substance that contains 6.02 x 10^23 particles (e.g. atoms or molecules).

Question 9

What is Avogadro’s constant?

Answer 9

The number of particles (e.g. atoms / molecules / ions) in 1 mole of a given substance. The value of the constant is 6.02 x 1023.

1 mole of carbon is exactly 6.02 x 1023 atoms of carbon.
1 mole of hydrochloric acid (HCl) is exactly 6.02 x 1023 molecules of HCl.

Question 10

What are the 3 forms of the formula that links mass, Mr, and moles?

Answer 10

mass = moles x Mr

moles = mass ÷ Mr

Mr = mass ÷ moles

Question 11

What do we mean by molar ratio?

Answer 11

The molar ratio is the ratio between the moles of different reactants and products in a balanced equation.

For example, in the following equation: N2+ 3H2➔ 2NH3
The ratio of N2 : H2 : NH3 is 1 : 3 : 2.

Question 12

What is the law of conservation of mass?

Answer 12

No atoms are lost or made during a chemical reaction.

Question 13

In a chemical reaction, is there any change in the overall mass of the substances?

Answer 13

No. The overall mass of the products will be exactly the same as the overall mass of the reactants.

Question 14

Give 3 reasons why the total mass may appear to change during a chemical reaction.

Answer 14

Gases from the air may react with the chemicals in the beaker, increasing the mass.

Gaseous products may escape from the beaker into the air, decreasing the mass.

Some chemicals may be lost via spilling or evaporation, decreasing the mass.

Question 15

What is a ‘limiting reactant’?

Answer 15

A reactant that all gets used up, and so limits how much product can be made (because there’s none of that reactant left).

Question 16

What does it mean if a reactant is ‘in excess’?

Answer 16

There is more than is needed, so there is some left over at the end of the reaction. In other words, there is an excessive amount.

Question 17

State the formula that links concentration, mass, and volume.
Give it in the form of ‘mass (g) = ? x ?’

Answer 17

mass (g) = concentration (g dm^-3) x volume (dm^3)

Question 18

What is the formula for calculating the concentration of a solution (using mass)?

Answer 18

concentration = mass ÷ volume

Question 19

What is the formula for calculating the volume of a solution (using mass)?

Answer 19

volume = mass ÷ concentration

Question 20

How do you convert cm^3 to dm^3?

Answer 20

divide the volume in cm^3 by 1,000

Question 21

What is the relationship between dm^3 and litres?

Answer 21

1 dm^3 = 1 litre

Question 22

A beaker contains 0.2 dm^3 of 2.5 g dm^-3 HCl.
Calculate the mass of HCl in grams.

Answer 22

mass (g) = concentration (g dm^-3) x volume (dm^3)

mass (g) = 2.5 g dm^-3 x 0.2 = 0.5 g

Question 23

Calculate the concentration of a solution containing 40 g of sodium hydroxide dissolved in 2.5 dm^3 of water.

Answer 23

concentration (g dm^-3) = mass (g) ÷ volume (dm^3)

concentration (g dm^-3) = 40 g ÷ 2.5 dm^3 = 16 g dm^-3

Question 24

A solution has a concentration of 35 g dm^-3.
What volume of solution in cm^3 would contain 14 g of solute?

Answer 24

volume (dm^3) = mass (g) ÷ concentration (g dm^-3)

volume (dm^3) = 14 g ÷ 35 g dm^-3 = 0.4 dm^3

volume (cm^3) = 0.4 × 1,000 = 400 cm^3

Question 25

State the formula that links the moles and volume of a gas. Write it in the form 'Volume = ? x ?'

Answer 25

Volume (dm^3) = moles x 24 Volume (cm^3) = moles x 24,000

Question 26

What is the volume of 1 mole of gas?

Answer 26

24 dm^3 or 24,000 cm^3.

Question 27

Would 5 moles of gaseous hydrogen and 5 moles of gaseous oxygen have the same volume?

Answer 27

Yes, they would have the same volumes. As long as the conditions remain the same, all gases occupy the same volume per mole.

Question 28

Define the term ‘atom economy’.

Answer 28

Atom economy is a measure of the proportion of starting materials that end up as useful products.

Question 29

What is the formula for atom economy?

Answer 29

Atom economy = Mr of desired products / Mr of all reactants x 100

Question 30

Is it better to have a high or low atom economy?

Answer 30

A high atom economy is better, as it means a greater proportion of reactants become useful products.

Question 31

What are the problems with low atom economy?

Answer 31

It will be more expensive as more reactants will be required. It will be less sustainable as it will use more resources. There will be more waste products, which will have to be disposed of (this could be expensive, or harm the environment).

Question 32

Name 6 other factors, besides atom economy, should be considered when choosing between two different methods of producing a useful product?

Answer 32

Percentage yield. Cost of raw materials. Position of equilibrium. Rate of reaction. Cost of maintaining the right conditions. Environmental impact of waste products.

Question 33

Without doing any calculation, what is the atom economy of this reaction: CaO + CO2 ➔ CaCO3

Answer 33

100% as there is only a single product. All reactions with a single product will have an atom economy of 100%.

Question 34

Explain in a sentence what the term ‘percentage yield’ means.

Answer 34

‘The actual yield of a reaction compared to the maximum theoretical yield’. Or: 'Percentage yield is a measure of how much product you actually produced, compared with how much you thought you should produce based on your calculations'.

Question 35

What is the formula for percentage yield?

Answer 35

% yield = (actual yield ÷ theoretical yield) x 100

Question 36

Suggest 3 reasons why the percentage yield of a chemical reaction may be less than 100%.

Answer 36

The reactants may not all react (e.g. because the reaction is very slow or reaches equilibrium). There may be side reactions, meaning that other products are produced instead. Some of the products may be lost (e.g. gases may float off, or some solids may be left on the filter paper).