Chapter 3 - Amount of substance

Question 1

What is a mole? And give the equation for it

Answer 1

Measure of amount of substance. To count the number of particles in a substance.

Moles = mass/Ar
or
Moles = mass/Mr

Question 2

What is the avogadro constant?

Answer 2

Gives the number of particles in one mole of a substance.
6.02 x 10^23 particles/molecules
Number of particles in one mole of carbon-12.

Question 3

What is the molecular formula?

Answer 3

Number of atoms of each element in a molecule.

Question 4

What is the empirical formula?

Answer 4

Simplest whole number ratio of atoms of each element in a compound.
Important for substances that do not exist as molecules e.g. ionic compounds or giant covalent.

Question 5

How do you find molecular formula?

Answer 5

Step 1: Convert % by mass or mass in grams into moles using moles = mass/mr
Step 2:Divide all the values for moles by smallest mole value to give you empirical formula.
Step 3: Divide molecular mass of compound by molecular mass of empirical formula to find how many unit of empirical formula are in the molecular formula.
Step 4: Multiply empirical formula by value found to give molecular formula.

Question 6

What is the water molecules in hydrated salts called?

Answer 6

Water of crystallisation.

Question 7

How do you work out the concentration of a solution?

Answer 7

The concentration of a solution is the amount of solute (in moles) dissolved in each 1 dm^3 of a solution.

Concentration (moldm^3) = number of moles/volume (dm^3)
1 moldm^3 solution contains 1 mole of solute dissolved in each 1 dm^3 of solution

Question 8

What is a standard solution?

Answer 8

A solution of known concentration

Prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume.

Question 9

What is the molar gas volume?

Answer 9

Volume per mole of gas molecules at a stated temperature and pressure

At RTP this is 24dm^3mol^-1

Question 10

How do you work out moles of a gas?

Answer 10

Amount of moles = volume/molar gas volume

Question 11

What are the standard conditions?

Answer 11

298K and 101Kpa (1atm)

Question 12

What is the ideal gas equation?

Answer 12

PV = nRT
Pressure (Pa)
volume (m^3)
number of moles of gas (mol)
ideal gas constant (8.314mol^-1k^-1)
temperature (K)

Question 13

What assumptions are being made for the molecules making up an ideal gas?

Answer 13

Random motion
Elastic collisions
Negligible size
No intermolecular forces

Question 14

What is stoichiometry?

Answer 14

Balancing numbers in balanced equations gives the ratio of amount, in moles, of each substance

Question 15

What do chemists use balanced equations to find?

Answer 15

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

Question 16

How do you find quantities from amounts and equations ? (reacting masses)

Answer 16

Step 1: Work out the amount of moles in whatever you can
step 2: Use equation to work out the amount in moles of the unknown chemical
step 3: work out unknown information

Question 17

What is percentage yield?

Answer 17

Conversion of reactants into desired product expressed as a percentage.

Percentage yield = actual yield/theoretical yield x 100%

Question 18

Why is theoretical yield hard to achieve?

Answer 18

Reaction may not go to completion
Side reactions alongside main reactions
Loss of product in separation/ purification/ handling
Reversible reaction

Question 19

What is the limiting reagent?

Answer 19

The reactant that is completely used up first and stops the reaction.

Question 20

What is atom economy?

Answer 20

Measure of how well atoms have been utilised

Question 21

What do reactions with high atom economies have as an advantage?

Answer 21

Produce larger proportion of desired products - and so few unwanted waste products. This is important for sustainability - making best use of natural resources.

Makes industrial uses more efficient, preserves raw materials and reduces waste.

Question 22

What is the definition of the term first ionisation energy?

Answer 22

Amount of energy required to remove one electron from one mole of gaseous atoms of an element, to form one mole of 1+ ions of an element.

Question 23

What happens to first ionisation energies down a group? And why?

Answer 23

Ionisation energies decrease down a group. This is because the atomic radius increases due to more electrons - meaning more shells - meaning more internal electron shielding, so nuclear attraction (between nucleus and outer electrons) decreases. Even though nuclear charge increases, this is outweighed by he electron shielding and distance.

Question 24

What happens to first ionisation energies across a period? And why?

Answer 24

Increases across a period as the atomic radius decreases due to nuclear charge increasing, as the shielding stays the same/ similar. This results in a stronger nuclear attraction (between nucleus and outer electrons).