Chapter 4 Chemical Calculations

Question 1

What is the relative atomic mass of an atom a comparison to?

Answer 1

The relative atomic mass of an atom is the mass relative to 1/12 of a carbon atom.

Question 2

How do you calculate the relative atomic mass (Ar) of an element?

Answer 2

The relative atomic mass (Ar) of an element is calculated by taking into account the percentage of all the isotopes of that element.

Question 3

What is an isotope?

Answer 3

An isotope is an atom of the same element but with a different number of neutrons.

Question 4

What is the relative formula mass of a compound (Mr) and how is it calculated?

Answer 4

The relative formula mass of a compound (Mr) is calculated by adding up all the relative atomic masses of all the atoms in the formula.

Question 5

What is the percentage by mass?

Answer 5

The percentage of a compound that is a single element can be calculated. This is called the percentage by mass.

Question 6

Describe how to calculate the % by mass of an element in a compound.

Answer 6

To calculate the % by mass of an element in a compound you must:
Step 1: Calculate the relative formula mass (Mr)
Step 2: Divide the mass of the element by the mass of the compound.
EXAMPLE

Question 7

Give the formula to calculate the number of moles in a given mass of a compound. (HIGHER ONLY)

Answer 7

The number of moles in a given mass of a compound can be calculated using the formula:

Question 8

List the steps for calculating the number of moles in a given mass of a compound. (HIGHER ONLY)

Answer 8

The steps for calculating the number of moles in a given mass of a compound are:
Step 1: Calculate Mr
Step 2: Convert the mass of the compound into grams
Step 3: Substitute into the equation

Question 9

What is the formula for calculating the mass of a compound from the number of moles and the relative formula mass. (HIGHER ONLY)

Answer 9

If you know the number of moles (n) required then the mass of the compound can be calculated using the formula:

Question 10

Give the steps for calculating the mass needed to get a given number of moles. (HIGHER ONLY)

Answer 10

The steps for calculating the mass needed to get a given number of moles is:
Step 1: Calculate Mr
Step 2: Substitute into the equation

Question 11

Why must equations always be balanced? (HIGHER ONLY)

Answer 11

Atoms cannot be created or destroyed so the number of atoms in the reactants must always be the same as the number of atoms of each atom in the products. Equations must be balanced.

Question 12

What are coefficients? (HIGHER ONLY)

Answer 12

We use coefficients to balance chemical equations. These are numbers in front of compounds. The chemical formula of a compound can never be changed when balancing.
Example: H2O2 → H2O + O2
2H2O2 → 2H2O + O2

Question 13

Detail the steps in balancing chemical equations.
(HIGHER ONLY)

Answer 13

To balance chemical equations:
Start with the largest element first and use coefficients to balance.

Balance the next largest element next.

Leave any elements on their own to the end (example: O2 or H2)

You may need to change coefficients along the way.

Always double check all elements balance before leaving the question.

Question 14

How should you lay out your answers to questions involving reacting masses and moles?
(HIGHER ONLY)

Answer 14

To calculate reacting masses, always layout your answers using the table format below.
Remember that you always need the formula mass (Mr) to convert between moles and mass and back again.
Take your time and complete each question step by step.

Question 15

Detail the steps to calculate reacting masses.
(HIGHER ONLY)

Answer 15

The steps to calculate reacting masses are:
Step 1: Write and balance the equation.
Step 2: Add the information you have.
Step 3: Find out the moles of the chemical with the given mass.
Step 4: Use the moles to work out the moles of the chemical you are looking for.
Step 5: Use the moles to work out the mass.

Question 16

Detail the steps in working out the balanced chemical reaction from reacting masses.
(HIGHER ONLY)

Answer 16

If the reacting masses are known then the balanced chemical equation can be worked out. The method is:
Convert the masses of the substances to moles.

Convert the numbers of moles to simple whole number ratios

Question 17

What do we need to do first before determining which reactants are in excess and which are limiting?
(HIGHER ONLY)

Answer 17

Balanced equations are needed to work out which reactants are limiting and which are in excess.

Question 18

Detail the steps for working out which reactant is limiting and which is in excess.
(TRIPLE ONLY)

Answer 18

To work out which reactant is limiting and which is in excess you need to:
Balance the chemical equation.

Use the mass given to work out the number of moles of each reactant.

Compare the ratio of moles to the molar ratio in the balanced chemical equation.

Question 19

Why do we calculate percentage yield?
(TRIPLE ONLY)

Answer 19

In real chemical reactions the mass of products often doesn’t match the mass predicted by calculations. Masses of products can be compared to the theoretical yield of products to calculate the percentage yield.

Question 20

What is the formula for calculating percentage yield?
(TRIPLE ONLY)

Answer 20

Percentage yield can be calculated using the equation:

Question 21

Give 4 Factors that reduce yield.
(TRIPLE ONLY)

Answer 21

Factors Reducing Yield
Product gets left behind in the apparatus.

Reversible reactions don’t go to completion

Some reactants for unexpected products or side reactions

Losses in separating the products from the reaction mixture.

Question 22

Give 3 factors that increase yield.
(TRIPLE ONLY)

Answer 22

Factors Increasing Yield
The product is contaminated.

Contamination can come from solvents used or the products of side reactions.

Question 23

What is atom economy?
(TRIPLE ONLY)

Answer 23

Atom economy, or atom utilisation, is the conversion efficiency or the extent to which atoms in the reactants end up in the desired product.

Question 24

What does atom economy indicate?
(TRIPLE ONLY)

Answer 24

Atom economy indicates how efficient a particular reaction is.

Question 25

What happens to products other than the intended product? (TRIPLE ONLY)

Answer 25

Other products from a reaction may be useful or they may be waste.

Question 26

Why is atom economy important in industry? (TRIPLE ONLY)

Answer 26

In industry atom economy is important for choosing which route to use to produce the required product most economically.

Question 27

What is the formula for atom economy? (TRIPLE ONLY)

Answer 27

Use the following equation to calculate atom economy:

Question 28

What is the formula for concentration of a solution?

Answer 28

The concentration of a solution can be calculated using the following equation:

Question 29

When calculating concentration of a solution what units must the volume must always be in?

Answer 29

When calculating concentration of a solution the volume must always be in dm3. This often involves converting from cm3. 1000 cm3 = 1 dm3

Question 30

Detail the method for making a solution of a known concentration?

Answer 30

The method for making a solution of a known concentration is: The mass of the solute is found on a scale The solute is washed into a volumetric flask with a small amount of solvent The solute is dissolved in a small amount of the solvent The flask is filled to the mark and inverted a few times with the stopper on to mix.

Question 31

How can we accurately determine the concentration of an unknown acid or alkali? (TRIPLE ONLY)

Answer 31

Titrations are used to accurately determine the concentration of an unknown acid or alkali.

Question 32

Detail the method for carrying out a titration. (TRIPLE ONLY)

Answer 32

The method for carrying out a titration is: Rinse a burette with water then with the acid you will use. Use a filter funnel to fill the burette above the 0 point. Run the excess acid out until the volume is exactly at the 0 point. Rinse a conical flask with water and a small quantity of the alkali of unknown concentration. Use a pipette to accurately measure a known volume (usually 20 cm3) of the unknown alkali into the conical flask. Add a few drops of an indicator (phenolphthalein or methyl orange). Add the acid slowly until close to the end point. When close to the end point add the acid one drop at a time. Record the volume of acid required to neutralise the alkali. This volume is called a titre. The first titre is called a rough titre and is used as a guide for the next run and not used in calculations. Repeat until there are at least 2 results within 0.010cm3 of each other. These results are described as concordant.

Question 33

Draw and label the equipment for setting up a titration. (TRIPLE ONLY)

Answer 33

Equipment for setting up a titration is as follows:

Question 34

Describe how to calculate the concentration of the alkali of unknown concentration. (TRIPLE ONLY)

Answer 34

To calculate the concentration of the alkali of unknown concentration: Use the volume of acid and the concentration of acid to calculate the number of moles of acid. Use the balanced equation to use the molar ratio to work out the moles of alkali. Use the moles of alkali and the volume to calculate the concentration using the formula:

Question 35

What is the volume of 1 mole of any gas. (TRIPLE ONLY)

Answer 35

1 mole of any gas has a volume of 24 dm3.

Question 36

What is the formula for calculating the volume of any gas. (TRIPLE ONLY)

Answer 36

To calculate the volume of any gas use the formula:

Question 37

Detail the steps in calculating the volume of gas of a product or reactant from a balanced chemical equation. (TRIPLE ONLY)

Answer 37

To calculate the volume of gas of a product or reactant from a balanced chemical equation: Calculate the number of moles of the given gas. Use the molar ratio to work out the number of moles of the gas required. Calculate the volume of gas.