# Understand Quantitative Chemistry

In this worksheet, students will learn about atomic and mass numbers, conservation of mass, reacting masses and reaction yield in the industry.

### 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 calcium oxide and carbon dioxide. 20g of calcium carbonate produced 11.2g of calcium oxide. How much carbon dioxide was made?

Answer: 20g of reactant must end with 20g 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) = 8.8g 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: 3g of magnesium react with oxygen to give 4.6g of magnesium oxide. How much magnesium oxide will 6g of magnesium give?

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

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

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.

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

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 products is equal to the mass of the reactants.

What would be the reason if, in an exam question, it was given that the mass of products is less than the mass of the reactants?

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

Write only the number in the answer box below.

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

Relative atomic mass of Ca: 40, O: 16

Write only the number in the answer box below.

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 5g of H if 10g of H gives 365g of HCl?

Write only the number in the answer box below (to one decimal place).

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

How much carbon dioxide is produced? Write your answer in grams, but only write the number in the box below.

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 5.5g, but it was predicted that 10g would be produced.

2.4g of magnesium gave 4g of magnesium oxide. How come the mass of the product is bigger than the mass of the reactant?

2g of oxygen reacted with magnesium

0.6g of oxygen reacted with magnesium

1.6g of oxygen reacted with magnesium

• Question 1

Define the term 'conservation of mass'.

The mass of products is equal to the mass of the reactants.
EDDIE SAYS
Mass of products is 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. Just like science to describe stuff that is obvious, right?
• Question 2

What would be the reason if, in an exam question, it was given that the mass of products is less than the mass of the reactants?

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

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

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

Write only the number in the answer box below.

100
EDDIE SAYS
You need to add the relative atomic mass of all the atoms that make up the compound. Note that there are three oxygen atoms in calcium carbonate.

40 + 12 + 16 + 16 + 16 = 100

Remember it is always advisable to show your calculations, because even if you may calculate wrongly, you can 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

Write only the number in the answer box below.

56
EDDIE SAYS
40 + 16 = 56
• Question 5

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

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 and 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 5g of H if 10g of H gives 365g of HCl?

Write only the number in the answer box below (to one decimal place).

182.5
EDDIE SAYS
If 10g of H gives 365g of HCl, then 5g would give half the amount, which is 182.5g of HCl.
• Question 7

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

How much carbon dioxide is produced? Write your answer in grams, but only write the number in the box below.

6.6
EDDIE SAYS
15 - 8.4 = 6.6g of carbon dioxide
• Question 8

Define the term 'yield of a reaction'.

The amount of chemical produced
EDDIE SAYS
The yield of a reaction is the amount of chemical produced. it is important to know this is you are making a specific chemical.
• Question 9

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

55
EDDIE SAYS

= 0.55 x 100 = 55%
• Question 10

2.4g of magnesium gave 4g of magnesium oxide. How come the mass of the product is bigger than the mass of the reactant?

1.6g of oxygen reacted with magnesium
EDDIE SAYS
1.6 g of oxygen reacted with magnesium. We know this because we only started with 2.4g of Magnesium and ended up with 4g of magnesium oxide. Since the only thing that we have added is oxygen, we must have added 1.6g (4-2.4) of oxygen.
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