Moles

Question 1

name the three subatomic particles

Answer 1

• protons
• neutrons
• electrons

Question 2

describe the relative charges and positions of each of the subatomic particles

Answer 2

• proton relative charge = +1
• proton position = nucleus
• neutron relative charge = 0
• neutron position = nucleus
• electron relative charge = -1
• electron position = electron shells

Question 3

define atomic number

Answer 3

the total number of protons in the nucleus of an atom

Question 4

define mass number

Answer 4

the total number of protons and neutrons in the nucleus of an atom

Question 5

define relative atomic mass

Answer 5

the weighted average mass of all the isotopes of an element compared to the mass of 1/12th of a ¹²C atom

Question 6

relative atomic mass formula

Answer 6

(mass x abundance) + (mass x abundance) etc.. / 100

Question 7

what is the symbol for relative atomic mass?

Answer 7

A_r

Question 8

how do you calculate the relative formula mass of a substance?

Answer 8

• find each of the relative atomic mass of each element present in the compound from the periodic table
• multiply each of these values by the amount of times they are present in the compound (e.g in H₂O, we’d times the RAM of hydrogen by 2 as there are 2 of them)
• add these values together to obtain the RFM

Question 9

what is the relative formula mass equivalent to?

Answer 9

• the relative formula mass (or the M_r) is equivalent to the molar mass (also the M_r) of a substance
• the RFM doesn’t have a unit whereas the molar mass is the same number as the RFM, just with the unit g/mol next to it

Question 10

what is the unit for amount of substance?

Answer 10

the mole (mol)

Question 11

what does one mole weigh exactly the same as?

Answer 11

one mole weighs exactly the same as the number of atoms in exactly 12g of carbon-12, which is 6.02 x 10²³

Question 12

what is avogadro’s constant?

Answer 12

6.02 x 10²³

Question 13

how do you find the number of atoms in a mole of an element or a compound?

Answer 13

• elements: RAM of element x avogadro’s constant
• compounds: RFM of element x avogadro’s constant

Question 14

how to calculate the % by mass of an element in a compound?

Answer 14

(RAM of element x number of its atoms present) / (RFM of compound) x 100

Question 15

moles formula

Answer 15

moles = mass/molar mass

Question 16

define empirical formula

Answer 16

the formula that shows the simplest whole number ratio of the atoms present in a compound

Question 17

define molecular formula

Answer 17

the formula that shows the actual number of atoms of each element present in a compound

Question 18

how do you calculate the empirical formula of a compound?

Answer 18

1. write the masses or percentages out in a ratio x:y
2. divide each number in the ratio by its RAM or M_r
3. divide each number in the ratio by the smallest number in the ratio to obtain the mole ratio
4. multiply the ratio until a whole number ratio is obtained
5. apply this ratio to the compound to obtain its empirical formula

Question 19

how do you calculate the molecular formula of a compound from its empirical formula?

Answer 19

1. calculate the RFM of the empirical formula
2. calculate a multiplier by doing RFM of molecular formula (given in Q dw) / RFM of empirical formula
3. multiply the given empirical formula by the multiplier to get the molecular formula

Question 20

what is the formula for the multiplier used in calculating the molecular formula of a substance from its empirical formula?

Answer 20

multiplier = RFM of empirical formula / RFM of given molecular formula

Question 21

what are waters of crystallisation?

Answer 21

water molecules which are part of a crystal structure in which they are chemically bonded to a salt

Question 22

what does the dot in the water of crystallisation mean?

Answer 22

loosely chemically bonded

Question 23

what part of the formula represents the waters of crystallisation? What is the correct ratio for salt:waters of crystallisation?

Answer 23

• the .nH₂O part where n shows the number of water molecules per unit of salt
• your final ratio of salt:water should be 1:x otherwise it’s wrong

Question 24

what is a hydrated salt?

Answer 24

a salt containing waters of crystallisation

Question 25

what is an anhydrous salt?

Answer 25

a salt where the waters of crystallisation have been removed

Question 26

how do you calculate the mass of water lost?

Answer 26

mass of hydrated salt - mass of anhydrous salt

Question 27

what is the method for producing magnesium oxide?

Answer 27

- COMBUSTION OF Mg METHOD - weigh the empty crucible w/o lid, and then w/ a ribbon of Mg in it - set up apparatus w/ bunsen burner, crucible and tripod + gauze - heat magnesium under a roaring flame and lift lid every few seconds to allow oxygen to enter to react w/Mg - once a white powder has formed, turn off bunsen burner and allow apparatus to cool - repeat the experiment X2 to heat contents to a constant mass to ensure experiment's done - weigh the apparatus again - there will be an increase in mass as Mg has gained mass of the oxygen to become MgO

Question 28

what are some areas for error when making MgO from Mg?

Answer 28

- some of the MgO powder may have escaped from the crucible when the lid was lifted = final mass decreases - not all Mg reacted with the oxygen so the increase in mass would be smaller than expected - lid not lifted frequently enough

Question 29

How do you calculate the empirical formula of magnesium oxide

Answer 29

- calculate the mass of Mg and mass of O - write out the mass of Mg to the mass of O as a ratio in the form x:y - divide each mass by its RAM - divide the ratio by the smaller number - multiply until you get a whole number ratio - apply the ratio to get MgO

Question 30

How do you calculate the mass of Mg and the mass of O₂ when calculating the empirical formula of magnesium oxide?

Answer 30

- Magnesium_mass = (initial mass of crucible + contents of Mg) - (mass of empty crucible) - Oxygen_mass = (final mass of crucible + contents) - (initial mass of crucible + contents of Mg)

Question 31

what is the method for producing copper from copper(II) oxide?

Answer 31

- REDUCTION OF COPPER(II) OXIDE METHOD - measure mass of empty reduction tube - set up apparatus containing a reduction tube w/methane supplied into it, a clamp + stand and a bunsen burner - flush out the reduction tube with methane to remove any oxygen to prevent an explosion - ignite the reduction tube + heat CuO for several minutes until it changes from a black powder to pink-brown metal - turn off the heat and flush out the tube with methane again to prevent the hot copper from reacting with any oxygen in the air

Question 32

what are some reasons for error in the reduction of copper(II) oxide experiment?

Answer 32

- reduction tube not flushed out with with methane well enough at the start so explosion occurs - reduction tube not flushed out with methane well enough at the end so final mass of copper would be greater than expected at hot copper would've reacted with oxygen in the air

Question 33

how do you calculate the empirical formula of copper(II) oxide from its reduction experiment?

Answer 33

- calculate the mass of copper and the mass of oxygen - place the mass of Cu to the mass of O in a ratio in the form of x:y - divide each element by its RAM - divide each number by the smallest one in the ratio - if answer is in decimals, multiply until a whole number ratio is obtained - apply the ratio to copper(II) oxide

Question 34

what is percentage yield?

Answer 34

percentage yield shows how much product was actually made in a reaction compared to how much was expected to be made

Question 35

what is theoretical yield?

Answer 35

the maximum mass of a product expected from a reaction

Question 36

what is actually being asked of you when maximum mass is mentioned?

Answer 36

theoretical yield

Question 37

what is actual yield?

Answer 37

the mass of product that is actually obtained from a chemical reaction

Question 38

what is the formula for percentage yield?

Answer 38

% yield = actual yield / theoretical yield x 100

Question 39

why is actual yield always lower than theoretical yield?

Answer 39

- gas can escape or leak from the equipment - gas may dissolve in water - reactants may be impure - spillages + loss of products during separation or purification processes (e.g filtration or distillation)

Question 40

what are the two types of reactants in an experiment?

Answer 40

excess and limiting reactant

Question 41

what is the excess reactant?

Answer 41

- the reactant that is in surplus - not all of it will react - some of it will be left at the end of the reaction, when the limiting reactant is used up

Question 42

what is the limiting reactant?

Answer 42

- the reactant that gets completely used up during the experiment - it determines how long the reaction lasts for and how much product is made

Question 43

what is avogadro's law?

Answer 43

equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

Question 44

at constant temperature and pressure, what is one mole of any gas defined as?

Answer 44

the volume occupied by any one mole of gas is approximately 24dm³ or 24000cm³, the molar gas volume

Question 45

formula for volume of gas

Answer 45

volume of gas = moles x molar gas volume

Question 46

how many cm³ are in one decimeter cubed?

Answer 46

1dm³ = 1000cm³

Question 47

if the Q is in cm³ or dm³, then how do you calculate the volume of gas?

Answer 47

- cm³ : volume = moles x 24000 - dm³ : volume = moles x 24