Chapter 3 - Amount of Substance

Question 1

What is Avogadro’s Constant?

Answer 1

The number of atoms per mole of the carbon-12 isotope.

Question 2

What is a mole?

Answer 2

The amount of any substance containing as many particles as there are particles in 12g of the carbon-12 isotope.

Question 3

What is the equation for the number of moles of a substance?

Answer 3

n=m/Mr
n - number of moles (mol)
m - mass (g)
Mr - molar mass (g/mol)

Question 4

What is relative formula mass?

Answer 4

The weighted mean mass of the formula unit of a compound compared with 1/12th of an atom of carbon-12

Question 5

What is the molecular formula?

Answer 5

The number of atoms of each element in a molecule.

Question 6

What is the empirical formula?

Answer 6

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

Question 7

How is the empirical formula found?

Answer 7

E   Element
M  Mass or %
---  ----------------
M  Molar mass
A   Answer (moles)
     ------------------------
     Smallest answer
R   Ratio

Question 8

How is the molecular formula found?

Answer 8

Find empirical formula.
Divide molar mass of a molecule by molar mass of empirical formula.
Multiply empirical formula by the result.

Question 9

What is relative molecular mass?

Answer 9

The mass of a molecule compared with the mass of an atom of carbon-12.

Question 10

In hydrated salts, what is the name given to water molecules?

Answer 10

Waters of crystallisation

Question 11

What is the concentration of a solute?

Answer 11

The amount of solute (in moles) dissolved in each 1dm^3 of solution

Question 12

What is the equation for finding a solution’s concentration?

Answer 12

c=n/v
c - concentration (mol/dm^3)
n - number of moles (mol)
v - volume (dm^3)

Question 13

What is molar volume?

Answer 13

The volume per mole of a gas.

Measured in dm^3/mol

Question 14

What is the value of molar volume at room temperature and pressure?

Answer 14

24 dm^3/mol

Question 15

What is the equation for molar volume?

Answer 15

mgv=V/n
mgv - molar gas volume (dm^3/mol)
n - number of moles (mol)
V - volume (dm^3)

Question 16

What are the assumptions made about an ideal gas?

Answer 16

Random motion
Elastic collisions
Negligible size
No intermolecular forces

Question 17

What is the equation for the ideal gas law?

Answer 17

pV=nRT
p - pressure (Pa)
V - volume (m^3)
n - number of moles (mol)
R - ideal gas constant (J/mol/K)
T - temperature (K)

Question 18

What are the standard conditions?

Answer 18

298K or 25C

101kPa or 1 atm of pressure

Question 19

What is stoichiometry?

Answer 19

The balancing numbers giving the ratio of the amount, in moles, of each substance in a balanced equation.

Question 20

What are the balanced equations used to find?

Answer 20

The quantities of reactants required to prepare a required quantity of a product.
The quantities of products that should be formed from certain quantities of reactants.

Question 21

What is the percentage yield?

Answer 21

The actual yield divided by the theoretical yield expressed as a percentage.

Question 22

Why is the theoretical yield so hard to achieve?

Answer 22

The reaction is incomplete.
Competing side reactions giving different products.
Loss of product in handling and purification.
Reversible reaction.

Question 23

What is the limiting reagent?

Answer 23

The reactant that is not in excess.

Question 24

What is atom economy?

Answer 24

The molar mass of desired products divided by the sum of the molar masses of all products expressed as a percentage.

Question 25

What are the benefits of a high atom economy?

Answer 25

Produce a larger proportion of desired products and few waste products.
Important for sustainability, the best use of natural resources.