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Understand Quantitative Chemistry

Worksheet Overview

QUESTION 1 of 10

Conservation of mass

When chemicals react, the mass of the reactants is always equal to the mass of the products. The total mass in a reaction never changes. If in an exam question, it is given that the mass has decreased, it will be because a gas was produced and it escaped. If it is given that the mass has increased, the reason will be that a gas, like oxygen, has reacted with the chemical.

Example question: When calcium carbonate is heated, it decomposes to make calcium oxide and carbon dioxide. 20 g of calcium carbonate produced 11.2 g of calcium oxide. How much carbon dioxide was made? 

Answer: 20 g of reactant must end with 20 g of product. We know that the products are calcium oxide and carbon dioxide, so:

20 (g of calcium carbonate)  - 11.2 (g of calcium oxide) = 8.8 g of carbon dioxide were produced.

 

Using simple ratio

When we know reactant and product mass for a reaction, we can use it to calculate different masses for the same reaction.

Example question: 3 g of magnesium react with oxygen to give 4.6 g of magnesium oxide. How much magnesium oxide will 6 g of magnesium give?

Answer: The given mass is 3 g and the question asks about double the mass, i.e. 6 g. Double the amount of magnesium will give double the amount of magnesium oxide; so, 4.6 x 2 = 9.2 g of magnesium oxide will be produced.

Extension question: How much oxygen was used in the reaction if 6 g of magnesium produced 9.2 g magnesium oxide?

Answer: 9.2 - 6 = 3.2 g of oxygen reacted with magnesium.

 

Yield and percentage yield of a reaction

The yield of a reaction is the amount of chemical that was produced in the reaction. The more reactant used, the more product is given. 100% yield means no product has been lost, whereas 0% yield means no product was made.

To calculate percentage yield, we use the following formula:

 

Formula for percentage yield

 

Relative atomic mass 

Atoms of different elements have different masses. The carbon card below is from the periodic table. Did you notice number 12 on the bottom? This is the relative atomic mass of carbon. Different periodic tables have them in different positions, but always look for the larger number between the two. 

 

Carbon on the periodic table.

 

Relative formula mass 

To calculate the relative formula mass of a compound, simply add up all the numbers of the elements in a formula. 

Example question: Calculate the relative formula mass of water. The formula for water is H2O. 

Answer: In an exam, you may be given the relative atomic masses of hydrogen (H) and oxygen (O) directly but you may just be given an extract from the periodic table and be expected to find them yourself. The relative atomic mass of hydrogen is 1 and for oxygen it is 16. Water has two atoms of hydrogen (H2) and one atom of oxygen (O), so:

relative formula mass of water = 1 + 1 + 16 = 18

 

Have another look at the information in this Introduction before moving on to the questions, just so you can be sure you have got your head around it all.

Define the term 'conservation of mass'.

The mass of reactants is always less than the mass of products

The mass of the products is always less than the mass of the reactants

The mass of the products is equal to the mass of the reactants

 In an exam question, it is given that the mass of the products is less than the mass of the reactants?

 

Why might this be?

A gas reacted with the initial element

A gas was produced

This is impossible, it could never happen

Calculate the relative formula mass of calcium carbonate (CaCO3).

 

Relative atomic mass of Ca: 40, C: 12, O: 16

A gas reacted with the initial element

A gas was produced

This is impossible, it could never happen

Calculate the relative formula mass of calcium oxide (CaO).

 

Relative atomic mass of Ca: 40, O: 16

A gas reacted with the initial element

A gas was produced

This is impossible, it could never happen

Where on the periodic table can you find the relative atomic mass of an element?

It is the smaller number on the element card

It is the larger number on the element card

It is the sum of the numbers on the element card

Hydrogen (H) and chlorine (Cl) react to give hydrochloric acid (HCl).

 

How much HCl will be produced from 5 g of H if 10 g of H gives 365 g of HCl?

 

Write your answer to 1 dp.

It is the smaller number on the element card

It is the larger number on the element card

It is the sum of the numbers on the element card

Calcium carbonate is broken down with heat to form calcium oxide and carbon dioxide. 15 g of calcium carbonate produces 8.4 g of calcium oxide.

 

How much carbon dioxide is produced?

It is the smaller number on the element card

It is the larger number on the element card

It is the sum of the numbers on the element card

Define the term 'yield of a reaction'.

The amount of gas that escaped

The amount of chemical produced

The amount of reactant used

Calculate the percentage yield of a reaction if the actual yield is 5.5 g, but it was predicted that 10 g would be produced.

2.4 g of magnesium gave 4 g of magnesium oxide.

 

How come the mass of the product is bigger than the mass of the reactant?

2 g of oxygen reacted with magnesium

0.6 g of oxygen reacted with magnesium

1.6 g of oxygen reacted with magnesium

  • Question 1

Define the term 'conservation of mass'.

CORRECT ANSWER
The mass of the products is equal to the mass of the reactants
EDDIE SAYS
This is a very important concept to grasp, so make sure you understand this before moving on. The correct answer is that the mass of the products will always be equal to the mass of the reactants. You can't make stuff out of nothing (not until you get to A-Level physics anyway) and conservation of mass is telling us this.
  • Question 2

 In an exam question, it is given that the mass of the products is less than the mass of the reactants?

 

Why might this be?

CORRECT ANSWER
A gas was produced
EDDIE SAYS
The reason would be that a gas had been produced and had escaped into the atmosphere. If it is in the atmosphere, then you can't measure its mass any more. This gives the impression that it has gone down in mass. Does that make sense?
  • Question 3

Calculate the relative formula mass of calcium carbonate (CaCO3).

 

Relative atomic mass of Ca: 40, C: 12, O: 16

CORRECT ANSWER
EDDIE SAYS
Don't be scared of these calculations - they really are not that hard and they are a great way to pick up some easy marks in the exam. You need to add the relative atomic mass of all the atoms that make up the compound. So that is one calcium, one carbon and three oxygen.
40 + 12 + 16 + 16 + 16 = 100 Remember, it is always advisable to show your calculations because even if you get the wrong answer, you might still gain some marks by showing that your thought process is correct.
  • Question 4

Calculate the relative formula mass of calcium oxide (CaO).

 

Relative atomic mass of Ca: 40, O: 16

CORRECT ANSWER
EDDIE SAYS
Did you get this one? Remember, simply add up the relative atomic masses of all of the atoms: 40 + 16 = 56
  • Question 5

Where on the periodic table can you find the relative atomic mass of an element?

CORRECT ANSWER
It is the larger number on the element card
EDDIE SAYS
It is the larger number on the element card. The smaller one is the atomic number, which is also the number of protons and electrons in the atom.
  • Question 6

Hydrogen (H) and chlorine (Cl) react to give hydrochloric acid (HCl).

 

How much HCl will be produced from 5 g of H if 10 g of H gives 365 g of HCl?

 

Write your answer to 1 dp.

CORRECT ANSWER
EDDIE SAYS
Just take these calculations one step at a time and you'll get there. If 10 g of H gives 365 g of HCl, then 5 g would give half the amount, which is 182.5 g of HCl.
  • Question 7

Calcium carbonate is broken down with heat to form calcium oxide and carbon dioxide. 15 g of calcium carbonate produces 8.4 g of calcium oxide.

 

How much carbon dioxide is produced?

CORRECT ANSWER
EDDIE SAYS
How are you doing with these calculations? Just take it slowly and if you're having trouble, then check out the Introduction once more. 15 g of calcium carbonate was the quantity of reactant we started with, so the products must also total 15 g. 15 - 8.4 = 6.6 g of carbon dioxide
  • Question 8

Define the term 'yield of a reaction'.

CORRECT ANSWER
The amount of chemical produced
EDDIE SAYS
Is this a new term for you to learn? It may sound a bit strange, but it is actually quite straight forward. The yield of a reaction is simply the amount of chemical produced. It is important to know what this is because you are making a specific chemical.
  • Question 9

Calculate the percentage yield of a reaction if the actual yield is 5.5 g, but it was predicted that 10 g would be produced.

CORRECT ANSWER
55
55%
EDDIE SAYS
This would be a hard question if we didn't know the formula for finding percentage yield! Can you remember it from the Introduction? percentage yield = actual yield ÷ predicted yield x 100 So, percentage yield = 5.5 ÷ 10 = 0.55 Then 0.55 x 100 = 55%
  • Question 10

2.4 g of magnesium gave 4 g of magnesium oxide.

 

How come the mass of the product is bigger than the mass of the reactant?

CORRECT ANSWER
1.6 g of oxygen reacted with magnesium
EDDIE SAYS
1.6 g of oxygen reacted with magnesium. We know this because we only started with 2.4 g of magnesium but ended up with 4 g of magnesium oxide. Since the only thing that we have added is oxygen, we must have added 1.6 g of oxygen because 4 - 2.4 = 1.6 Well done for tackling all these calculations -are you finding them easier to do with practice?
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