Quantitive

Question 1

Relative formula mass (Mr)

Answer 1

The sum of the relative atomic masses of all the atoms present in the formula of a substance

Question 2

Equation for % element in a compound (% composition)

Answer 2

% element A = Ar or A x number of atoms of A in compound
—————————————————————— X100
Mr of whole compound

Question 3

Mole

Answer 3

One of a substance contains 6.02x10^23 atoms/molecules of that substance

Question 4

Molar mass

Answer 4

The mass of one mole of the substance and is numerically equal to the RAM/RFM in grams

Question 5

Mole equation

Answer 5

Mass (g)
—————
Molar mass

Question 6

Actual yield

Answer 6

The mass of the product measured at the end of the reaction

Question 7

Theoretical yield

Answer 7

The max yield which can be produced using the calculation using a balanced chemical equation for the reaction

Question 8

% yield

Answer 8

Actual yield
———————- x 100
Theoretical yield

Question 9

Limiting reactant

Answer 9

The reactant which runs out first in a reaction, limiting the amount of product that can be made

Question 10

Empirical formula

Answer 10

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

Question 11

Molecular formula

Answer 11

The actual number of atoms in each element in a molecule

Question 12

Water of crystallisation

Answer 12

Water that is chemically bonded into the crystal structure

Question 13

Anhydrous hydrated dehydration

Answer 13

Doesn’t contain water of crystallisation
Solid crystals contain water of crystallisation
Removal of water of crystallisation

Question 14

heat to constant mass

Answer 14

The process of heating and weighing is repeated until the mass stops changing. This ensures that the reaction is complete

Question 15

Degree of hydration

Answer 15

The number of moles of water of crystallisation chemically bonded in 1 mole of the compound

Question 16

What is a weighted mean , why is it used to calculate Ar

Answer 16

It takes into account the isotopes of that element and their contribution to the overall mass of the element

Question 17

Relative atomic mass units

Answer 17

NO UNITS

Question 18

What is 1 mole of any substance and what is it known as

Answer 18

6.02x10^23 , avogadro’s constant

Question 19

What do we do after weighing out the Mr in grams

Answer 19

We have 1 mole of the particle represented by that formula , therefore we can count particles by weighing them

Question 20

What do you do if you’re asked to find what 1 mole of a substance weighs , molar mass

Answer 20

Find it’s Mr , stick a “g” sign after it

Question 21

1 tonne in grams

Answer 21

1,000,000g

Question 22

1kg in grams

Answer 22

1000g

Question 23

Calculating reacting masses using chemical equations

Answer 23

Find moles
Move them using ratio
Find mass that is asked for

Question 24

What do we do to work out the limiting reactant

What are the other reactant described as

Answer 24

We use the mole ratio from the balanced chemical equation

Being in excess

Question 25

Calculations involving a limiting reactant What will be given Method

Answer 25

Masses of more than 1 reactant Work out which is in excess and then use the other , to calculate mass of product Calculate moles of each substance Ratio them Find the smallest Convert to mass

Question 26

What does % yield tell us

Answer 26

Gives a measure of how efficient the chemical process is

Question 27

Why is % yield never 100%

Answer 27

Competing side reactions This is due to other chemical reaction that may also take place Physical loss of product in separation from the reaction mixture eg: - loss in transfer from 1 piece of apparatus to another - some product may be left on filter paper during filtration - some product may stay in solution after crystallisation Reaction is reversible and doesn’t go to completion

Question 28

Empirical formula by experiment

Answer 28

The word empirical = determined by experiment , by determining the empirical formula of a wide range of compounds scientists came up with the rules about VALENCY that we now use to work out formula

Question 29

Empirical formula working out

Answer 29

X Y Z Mass Mr Moles Ratio

Question 30

How to find molecular formula hydrocarbon , Mr = 42 , EP= CH2

Answer 30

From empirical formula if you know the Mr of the compound Add up the atomic masses of the atoms in empirical formula The mass of the atom in the empirical formula is eg 14 42/14 = 3 So you need to multiply the numbers in the empirical by 3 therefore hydrocarbon is C3H6

Question 31

Finding the formula of copper oxide What does the hydrogen do How to work out the mass of O2 in CuO What can be calculated from these values

Answer 31

Passed over heated copper oxide it reduces it, taking away oxygen to leave copper By measuring the initial mass of the copper oxide and the final mass of copper obtained Formula of copper oxide

Question 32

Finding out the formula of copper oxide Observations HOW DOES ALL THE COPPER OXIDE BEEN REDUCED

Answer 32

Black solid (CuO) turns red-brown Solid condensation initially appears on the cooler part of the tube After heating apparatus must be allowed to cool down before the supply of hydrogen gas is turned off as the hot metal would reoxidise into copper oxide HEATING TO A CONSTANT MASS , this is the process of heating and weighing until the mass stops changing, ensures that the reaction is complete

Question 33

Finding the empirical formula of MgO

Answer 33

Cut a piece of magnesium about 5 cm long , if looks black or tarnished then clean it using emery paper, twist it into a loose coil Weigh the crucible with the lid and then the magnesium inside the crucible with the lid Light the bunsen and begin to heat the crucible Once the crucible is hot gently lift the lid using tongs a little, to allow some oxygen to get in you may see the magnesium begin to flare up (If the lid is off for to long then the magnesium oxide product will begin to escape don’t let this happen) Keep heating and lifting with the lid until you see no further chemical reaction At this point remove the lid and heat for another couple of minutes , replace the lid if it appears you are loosing some product Turn off the Bunsen and allow apparatus to cool Re-weigh the crucible with the lid containing the product Heat the crucible again for another couple of minutes and allow to cool again Repeat this step until the mass reading is consistent called “heat to a constant mass”

Question 34

Test for water

Answer 34

When water is added to anhydrous copper(II) sulfate it turns from a white powder to blue

Question 35

Degree of hydration CuSO4.5H2O

Answer 35

The 5 The number of moles of water of crystallisation chemically bonded in 1 mole of the compound

Question 36

Calculating Mr and the % of water for hydrated salts What do some ionic compounds contain Eg CuSO4.5H2O contains,……… water molecules Mr = 250

Answer 36

Water of crystallisation, this must also be counted when calculating Mr 5 %water = 90/250 x100 =36%

Question 37

Working out the formula of a compound containing water of crystallisation experimental results What to do

Answer 37

Take away hydrate from anhydrous to get H2Og and anhydrous is the other g X H2O g Mr Moles Ratio Formula

Question 38

Determining the formula of a compound containing water of crystallisation experimentally How to calculate 1 the mass of the anhydrous salt 2 the mass of the water in the crystals

Answer 38

We need to take a sample of hydrated salt and heat to drive off the water of crystallisation. In the experiment we need to find 1 and 2 The residue left after heating The decrease in mass on heating

Question 39

Finding the mass of water in a hydrated compound

Answer 39

Weigh and record mass of crucible Add around 25g of hydrated compound, weigh and record mass of crucible and hydrated compound Place the crucible in a pipe clay triangle and heat to remove the water of crystallisation Allow the crucible to cool, then weigh and record mass Repeat the heating, cooling a and reweighing until two masses are the same (heat to constant mass)

Question 40

Finding the formula of hydrated CuSO4

Answer 40

Weigh empty crucible Weigh into it 2-3g of hydrated CuSO4 , record all weighings accurate to the nearest 0.01g Set up apparatus as shown Heat the crucible and contents gently at first,over a medium Bunsen flame so that the water of crystallisation is drove of steadily . The blue colour of hydrated compound should gradually fade into an anhydrous greyish white. Avoid over heating which may cause further decomposition and stop heating , toxic or corrosive fumes may be evoked. A total heating time of about 10minutes should be enough Allow crucible and contents to cool, the tongs may be used to remove the crucible from the hot clay triangle onto the heat resistant mat where it should cool more quickly Re-weigh the crucible and its contents once cold