Physical 1: Amount of Substance

Question 1

relative atomic mass (Ar)

Answer 1

relative atomic mass = (average mass of one atom of an element) / (1/12 mass of one atom of 12C)
relative atomic mass = (average mass of one atom of an element) / (mass of one atom of 12C)

Question 2

relative molecular mass (Mr)

Answer 2

relative molecular mass = (average mass of one molecule) / (1/12 mass of one atom of 12C)
relative molecular mass = (average mass of one molecule x 12) / (mass of one atom of 12C)

Question 3

relative formula mass (Mr)

Answer 3

used for ionic compounds because they don’t exist as molecules

Question 4

avogadro constant

Answer 4

6.022x10^23

it is the number of atoms in 12g of carbon-12

Question 5

the mole

Answer 5

the amount of substance that contains 6.022x10^23 particles is called a mole

Question 6

number of moles formula

Answer 6

number of moles = mass / mass of 1 mole

Question 7

concentration formula

Answer 7

concentration = mass / volume

Question 8

Boyle’s Law

Answer 8

pressure x volume = constant

Question 9

Charles’ Law

Answer 9

volume / temperature = constant

Question 10

Gay-Lussac’s Law (constant volume law)

Answer 10

pressure / temperature = constant

Question 11

ideal gas equation

Answer 11

PV = nRT 
P = pressure Pa 
V = volume m^3
n = number of moles 
R = gas constant = 8.31 JK-1mol-1
T = temperature K

Question 12

rearrange ideal gas equation for volume

Answer 12

V = nRT / P

Question 13

rearrange ideal gas equation for number of moles

Answer 13

n = PV / RT

Question 14

empirical formula

Answer 14

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

Question 15

how to find empirical formula

Answer 15

1. find masses of each of the elements present in a compound
2. work out the number of moles of atoms of each element :
   number of moles = (mass of element) / (mass of 1 mol of element)
3. convert the number of moles of each element into a whole number ratio

Question 16

molecular formula

Answer 16

actual number of atoms of each element in one molecule of the compound
empirical is not always the same as the molecular

Question 17

how to find the number of units of the empirical formula in the molecular formula

Answer 17

divide the relative molecular mass by the relative mass of the empirical formula

Question 18

stoichiometry

Answer 18

the ratio in which the reactants react and the products are produced in simple whole numbers

Question 19

titration steps

Answer 19

1. fill a burette with the acid of known concentration
2. accurately measure an amount of the alkali using a calibrated pipette and pipette filler
3. add the alkali to a conical flask with a few drops of a suitable indicator
4. run in acid from the burette until the colour just changes, showing that the solution in the conical flask is now neutral
5. repeat the procedure, adding the acid dropwise as you approach the end point, until two values of the volume of acid used at neutralisation are the same, within experimental error

Question 20

atom economy

Answer 20

% atom economy = 100 x (mass of desired product) / (total mass of reactants)

Question 21

atom economy vs percentage yield

Answer 21

atom economy = in theory how many atoms must be wasted in a reaction
percentage yield = tells us about the practical efficiency of the process, how much is lost by:
- the practical process of obtaining a product
- as a result of reactions that do not go to completion

Question 22

formula of percentage yield

Answer 22

yield = 100 x (no. of moles of specific product) / (theoretical max no. of moles of product) 
yield = 100 x (no. of grams of specific product) / (theoretical max no. of grams of product)