C3 Quantitative chemistry

Question 1

What is the law of conservation of mass ?

Answer 1

The law of conservation of mass states that no atoms are created or destroyed during a chemical reaction so the mass of the products equals the mass of the reactants.

Question 2

What is the relative formula mass of a compound ?

Answer 2

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

Question 3

How can a chemical reaction result in an increase in mass ?

Answer 3

For example: when a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal

Question 4

Why can some reactions involve a loss in mass ?

Answer 4

If a reactant or product is a gas. For example in the thermal decompositions of metal carbonates, carbon dioxide is produced and escapes into the surroundings leaving the metal oxide as the only solid product.

Question 5

In the reaction Zn + 2HCl—–> ZnCl2 + H2 explain what happens to the overall mass in the reaction if it happens in an unsealed container.

Answer 5

The mass decreases as hydrogen is a gas and escapes into the surroundings.

Question 6

What is the symbol for moles ?

Answer 6

mol

Question 7

What are chemical amounts measured in ?

Answer 7

moles

Question 8

What is the mass of 1 mole of a substance if it is an element in grams ?

Answer 8

The mass of one mole of an element in grams is equal to its relative atomic mass (Ar).

Question 9

What is the mass of 1 mole of a substance if it is a compound in grams ?

Answer 9

The mass of one mole of a compound in grams is equal to its relative formula mass (Mr).

Question 10

What is the avagadro constant ?

Answer 10

The number of atoms, molecules or ions in one mole of a given substance

Question 11

What is the value of Avagadro’s constant ?

Answer 11

The value of the Avogadro constant is 6.02 x 10^23 per mole.

Question 12

How do you calculate the number of moles of an element/compound?

Answer 12

moles = m/Mr

m = mass of substance
| Mr = molar mass

Question 13

How do you calculate the number of atoms in a given substance?

Answer 13

Number of atoms = Avogradro constant x the amount of substance in moles

Question 14

What is the mass of 1 mole of oxygen gas

Answer 14

32 grams

Question 15

What is the mass of 1 mole of oxygen atoms?

Answer 15

16 grams

Question 16

What is the mass of 1 mole of carbon dioxide.

What is the mass of 0.25 moles of carbon dioxide?

Answer 16

44 grams

11 grams

Question 17

How many moles of carbon dioxide are there in 88 grams of carbon dioxide.

Answer 17

2 moles

Question 18

How many moles are in 10 grams of calcium carbonate

Answer 18

0.1 moles

Question 19

Write an equation for the thermal decomposition of Zinc carbonate.

Answer 19

ZnC03———-> ZnO + CO2

Question 20

When 3.1 grams of zinc carbonate undergoes thermal decomposition calculate the mass of ZnO produced.

Answer 20

1. mass of ZnCO3 = 3.1grams
2. moles of ZnCo3 = 3.1/125 grams = 0.0248moles
3. 1 mole ZnO is 81g (molar mass) therefore 0.0248 moles = 2.0088 grams
4. 2.0088 grams = 2.01g (2d.p)

Question 21

In a reaction between magnesium and HCl, Anya wanted to find out how changing the amount of magnesium affected the rate of the reaction. Explain why the HCL needs to be in excess in this reaction and why the magnesium is called the limiting factor.

Answer 21

To make sure that HCL is not changing the reaction, the HCL needs to be in excess. This makes sure that the reaction does not stop because the HCL is finished. It is only the amount of magnesium that is limiting the reaction.

Question 22

Calculate the mass of sodium chloride needed to make 0.5 litres of a 2 molar solution of sodium hydroxide?

Answer 22

moles = 0.5 x2 =1 mole

1 mole = to 58.5grams

Question 23

Calculate the mass of calcium needed to make 25 cm3 of a 2 molar solution

Answer 23

moles = 2 x 0.025 = 0.05 mole

1 mole = 100grams 0.05 = 5 grams

Question 24

Write down 3 ways that might cause all the reactants not to turn into products.

Answer 24

the reaction may not go to completion because it is reversible
• some of the product may be lost when it is separated from the reaction mixture
• some of the reactants may react in ways different to the expected reaction.

Question 25

What is the percentage yield?

Answer 25

The amount of a product obtained is known as the yield. When compared with the maximum theoretical amount as a percentage, it is called the percentage yield.

Question 26

What is atom economy ?

Answer 26

It is a measure of the amount of starting materials that end up as useful products.

Question 27

Why is it important to have a high atom economy

Answer 27

It is important for sustainable development and for economic reasons to use reactions with high atom economy.

Question 28

How is the percentage atom economy of a reaction measured

Answer 28

The percentage atom economy of a reaction is calculated using the balanced equation for the reaction as follows:
Mass of useful product/mass of all the reactants X 100

Question 29

When do equal amounts of gases in moles occupy the same volume ?

Answer 29

Equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure.

Question 30

What volume does 1 mole of gas occupy att r.t.p

Answer 30

The volume of one mole of any gas at room temperature and pressure (20oC and 1 atmosphere pressure) is 24 dm3.

Question 31

Calculate the volume of gas produced when 12 grams of Magnesium metal reacts with excess HCL

Answer 31

12 dm3

Question 32

Calculate volume of gas produced when 2 grams of CaCO3 reacts with excess HCL

Answer 32

1.2 grams

Question 33

Calclate mass of CaCO3 needed to react with excess HCL to produce 6dm3 of gas ?

Answer 33

this is a 1/4 of a mole of gas from the equation we need a 1/4 of a mole of CaCo3 1/4 x 100 = 25grams