Amount of Substance

Question 1

What is relative atomic mass?

Answer 1

The weighted average mass of all the isotopes relative to 1/12th the mass of one atom of carbon-12.

Question 2

How do you calculate relative atomic mass?

Answer 2

Sum of (mass X abundance of each isotope)
————————————————————
Total abundance

Question 3

What is relative molecular mass?

Answer 3

The mass of a molecule of the compound relative to the 1/12th of the mass of an atom of carbon-12.

Question 4

What is relative formula mass?

Answer 4

The mass of one formula unit of an ionic compound relative to 1/12th of the mass of an atom of carbon-12.

Question 5

What is a mole?

Answer 5

An amount of substance that contains as many particles as there are in exactly 12g of carbon-12.

Question 6

What is the equation for the number of moles that includes mass and Mr?

Answer 6

Mass = Mr X Moles

Question 7

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

Answer 7

1) Work out the number of moles.
2) Number of atoms = number of moles X Avogadro’s constant

Question 8

What is the equation for the number of moles that includes volume and concentration?

Answer 8

Number of moles = Volume X Concentration

Question 9

What is the ideal gas equation?

Answer 9

PV = nRT

Pressure X Volume = Moles X Gas constant X Temperature

Question 10

What assumptions are made in the ideal gas equation?

Answer 10

• There are no intermolecular forces between the molecules.
• The volume occupied by the molecules themselves is entirely negligible relative to the volume of the container.

Question 11

What is the equation for density?

Answer 11

Density = Mass / Volume

Question 12

What is the empirical formula?

Answer 12

The simplest whole number ratio of atoms of each element present in a compound.

Question 13

What is molecular formula?

Answer 13

The actual number of atoms of each element present in a molecule.

Question 14

What does stoichiometry tell you?

Answer 14

The relative number of moles of reactants and products involved in a reaction.

Question 15

How do you work out the mass of a product when given the mass of the reactant?

Answer 15

• Work out moles of the reactant.
• Write the balanced equations to work out the stoichiometry.
• Work out the mass by using the stoichiometry ratio and moles equation.

Question 16

How do you make a standard solution?

Answer 16

1) Record the mass of a weighing boat and the solid.
2) Tip the solid into a beaker and record the mass of the weighing boat and the residue.
3) Determine the mass of the solid used by calculating the difference between the two masses. This is called weighing by difference.
4) Dissolve the solid in deionised water.
5) Using a funnel, transfer into a 250cm3 volumetric flask and include washings.
6) Make up to the graduated line by carefully adding deionised water, making sure the meniscus sits on the line.
7) Put a stopper in the flask and invert several times to mix the contents thoroughly.

Question 17

How do you find the concentration of a solution of a base (titration)?

Answer 17

1) Fill a burette with a standard solution of the acid.
2) Pipette a known volume of base into a conical flask.
3) Add a few drops of indicator to the base in the flask.
4) Add the solution of the acid from the burette until the indicator just changes colour. This is known as the end-point. This process is known as a rough titration.
5) Record the volume of acid added. Make sure to record the burette readings to the nearest 0.05cm3.
6) After performing a rough titration, perform accurate titrations until they are concordant (within 0.10cm3 of each other).
7) Calculate the mean titre from the concordant titres.

Question 18

How do you calculate percentage error?

Answer 18

Uncertainty
—————— X 100
Value

Question 19

How do you calculate the number of moles of water of crystallisation?

Answer 19

• Mass of water = mass of hydrated salt — mass of anhydrous salt.
• Work out the number of moles of anhydrous salt and of water.
• Fine the simplest whole number ratio.

Question 20

How do you calculate percentage yield?

Answer 20

Actual Yield of Product
——————————— X 100
Theoretical Yield of Product

Question 21

How do you calculate atom economy?

Answer 21

Molecular mass of desired product
————————————————— X100
Sum of molecular masses of all products