4.4 - Moles I

Question 1

what is the relative formula mass/ RFM

Answer 1

the mass of a formula unit of substance

Question 2

how is RFM calculated

Answer 2

by adding the relative atomic masses of the atoms or ions

Question 3

what does percentage by mass show

Answer 3

the mass of a substance that comes from a particular element

Question 4

what is the equation for percentage by mass

Answer 4

%(x) = mass of (x) / RFM x100

Question 5

what is a mole

Answer 5

the unit of amount of substance

Question 6

what do moles represent

Answer 6

how many particles of a substance there are, taking into account particles of different substances have different masses

Question 7

what is the equation for calculating moles

Answer 7

moles = mass(g) / RFM

Question 8

what is the empirical formula

Answer 8

contains the simplest mole ratio of its elements

Question 9

how do you calculate the empirical formula

Answer 9

1- convert masses or percentages into moles
2- cancel down the mole ratio
3- write the simplest mole ratio into a formula

Question 10

what should you draw to calculate the empirical formula

Answer 10

mass, moles, ratio table

Question 11

what is the molecular formula

Answer 11

contains the actual numbers of atoms in a molecule - like the empirical formula but uncancelled

Question 12

how do you calculate the molecular formula the quick way for example using the empirical formula CH2 and RFM 70

Answer 12

RFM of CH2 = 14
70/14 = 5
molecular formula = 5 x CH2 = C5H10

Question 13

how is water crystalisation written into a formula

Answer 13

CuSO4(salt) . 5H2O (water of crystalisation)

Question 14

what is the mole ratio always in the form of

Answer 14

1 : x

Question 15

what is water of crystalisation calculated from

Answer 15

mases of salt and water produced on heating

Question 16

how would you calculate ‘x’ if 16.7g of MgCl2 . xH2O crystals decreased in mass to 9.5g on heating

Answer 16

1- calculate the mass of water lost
—– 16.7 - 9.5 = 7.2g
2- draw a mass, moles, ratio table

Question 17

what do the balancing numbers in a balanced equation for a reaction tell us

Answer 17

how many moles of each reactant are needed
how many moles of each product are made

Question 18

what is a limiting substance

Answer 18

if there isn’t enough

Question 19

what is an excess substance

Answer 19

if there is more than enough

Question 20

what is the calculation for percentage yield

Answer 20

%yield = experimental(from experiment)/ theoretical (from calculation) x 100

Question 21

what is the experimental mass measured in

Answer 21

an experiment or given to you in the question

Question 22

what is the theoretical mass based on

Answer 22

the mass of the reactant

Question 23

what are the steps to calculating percentage yield

Answer 23

1- find or make a balanced equation for reaction
2- calculate theoretical mass that should have been produced
3- calculate the percentage yield using experimental mass from question