3.1.2 Amount of substance

Question 1

Relative atomic mass Ar

Answer 1

The weighted average mass of an atom of an element , taking into account its naturally occurring isotopes relative to 1/12th of a carbon 12 atom

Question 2

Relative molecular mass Mr

Answer 2

Mass of a molecule of that molecules compared to 1/12th the relative atomic mass of an atom of carbon 12

Question 3

Avogadro’s constant

Answer 3

The number of atoms in 12g of carbon-12

Question 4

Mole

Answer 4

amount of substance that contain 6.022 x 10^23 particles

Question 5

Mol, mass, mr equation

Answer 5

Mol = mass/mr

Question 6

Mol, concentration,volume

Answer 6

Concentration = mol / vol

Question 7

No of particles eqs

Answer 7

= Mol of substance x avogadro’s constant

Question 8

Density

Answer 8

Mass / volume
Normally to do with pure liquids to work out the mass from measured volume - and used in solids and gases

Question 9

Ion dissociation

Answer 9

When soluble ionic solids dissolve in water they dissociate into separate ions - lead to concentration of ions differing from the concentration of solute

Question 10

Calculating dilutions

Answer 10

New diluted conc = original conc x original vol ;/ diluted vol

Question 11

Volume

Answer 11

Changes with pressure and temperature

Question 12

Ideal gas equation

Answer 12

PV = nRT

Question 13

Units for PV = nRT

Answer 13

• P = Pa
• V = m cubed
• n = moles
• R = gas constant (8.31)
• T = K

Question 14

Conversion between degrees c and K

Answer 14

Add 273 to degrees c

Question 15

conversion between cm cubed and m cubed

Answer 15

divide by a million 100 000

Question 16

Changing the conditions of a gas

Answer 16

1. Workout mols of gas using ideal gas equation then but in new equation
   Or combine them - put equal to each other after rearranged for n

Question 17

Mol of a gas at room temp

Answer 17

1 mol of any gas at 1atm and at 25 degrees c will have the volume of 24dm cubed

Question 18

Empirical formula

Answer 18

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

Question 19

How to calculate empirical formula

Answer 19

1. Divide each mass / percentage mass by the atomic mass of the element
2. For each of the answers in stem 1 divide by the smallest one of those number
3. Sometimes the numbers calculated in step 2 will need to be multiplied up to give a whole number
   Whole numbers = the empirical formula

Question 20

How to calculate molecular mass

Answer 20

Mr of molecular formula/ the mr of the empirical formula
Multiplied by the the number of each element present in the empirical formula

Question 21

Ionic equations

Answer 21

Split the aq component into its ions

Remove the spectator ions (appear in both the reactants and the products)

Question 22

Atomic economy

Answer 22

Percentage atomic economy = mass of desired product / total mass of reactants x100

Question 23

Percentage yield equation

Answer 23

Percentage yield = actual / theoretical x 100

Question 24

High percentage yield and atom economy

Answer 24

High percentage yield = efficient conversion of reactants to products
High atom economy = max mass of reactants in desired product

Question 25

Percentage uncertainty

Answer 25

Uncertainty / reading x 100

Question 26

Reducing percentage uncertainty in a titration

Answer 26

To reduce the conc of the substance in the burette
Meaning more is needed to neutralise meaning larger measurement

Question 27

calculating the percentage mass of useful stuff in a sample contaminated method

Answer 27

1. workout the mols in sample (of known in burette) reaction stuff in the burette
2. map that to mols of what was being neutralized in titration
3. then scale that up to the size of the sample that the small vol was taken out of
4. calculate mass of sample that has reacted
5. workout the difference in mass from that reacted and stated
6. do difference/ initial x 100