QUANTITATIVE CHEM

Question 1

how to work our relative formula mass of compound/molecule
how do you work out % mass of an element in a compound

Answer 1

add up relative atomic masses of all atoms
relative atomic mass of element (Ar)/ relative formula mass of compound(Mr) x100

Question 2

Calculate percentage by mass of H in CH₄
H Ar=1 CH₄ Mr= 16

Answer 2

1x4 / 16 x 100 = 25%

Question 3

how many g in a tonne

Answer 3

1000000

Question 4

what is avagadros constant
What is a mole of any substance

Symbol for moles

Answer 4

6.02x10²³
an amount of substance that contains 6.02x10²³ particles- atoms, molecules,ions, electrons
mol

Question 5

Why is Avagadro Constant used

Answer 5

one mole of a substance will have mass EQUAL to Ar / Mr of that substance
eg
N₂ has Mr of 28. One mole of N₂ is 28g
C Ar= 12, One mole of C is 12g
CO₂ Mr= 44 One mole of CO₂ is 44g
12 g of Carbon, 28g of N₂, 44g of CO₂ all contain one mole (6.02x10²³) atoms/molecules

Question 6

formula triangle for number of moles in a given mass

Answer 6

——mass——
Mr——–mol

Question 7

state law of conservation of mass

Answer 7

no atoms lost/made during chemical reaction.
so mass of products= mass of reactants (mass is concerved)

Question 8

why would there be a change in mass in an unsealed vessel
An increase-
3 points

A decrease-
4 points

Answer 8

reactant/product is gas

-A REACTANT is a gas.
-BEFORE reaction, gas is floating, not contained in vessel, mass NOT taken into account
-Gas reacts to form product, its contained inside vessel, TOTAL mass inside vessel INCREASES

-A PRODUCT is gas,
-REACTANTS are s/ l / aq.
-BEFORE reaction, all reactants contained in vessel.
-Gas escapes as its formed. Gas X in vessel-Mass of gas X taken into account.- TOTAL mass inside vessel DECREASES

Question 9

eg of when
mass seems to increase
mass seems to decrease
(due to gasses)
Equations for these with State symbols

Answer 9

• Metal + Oxygen react —-> MetalOxide produced w/ greater mass than metal. (o₂ floating in air, mass not accounted)
  -mass of MetalOxide = TOTAL mass of reactants
  Metal(s) + Oxygen (g) = Metal oxide(s)

-Metal carbonate thermally decomposes to form Metal Oxide + CO₂
-CO₂ escapes into atmosphere
-metal oxide is the only solid product
Metal carbonate(s) = Metal oxide(s) + Carbon Dioxide (g)

Question 10

why does mass in a unsealed container increase/ decrease if Gas produced in the reaction.

Answer 10

Gases expand to fill any container. If vessel unsealed, gas expands out from vessel, escapes into air

Question 11

a mixture contains 20% iron ions by mass.
What mass of iron Chloride (FeCl₂) would be needed to provide the iron ions in 50 g of the mixture. Ar of Fe=56 Cl=35.5

Answer 11

-Find mass of iron in mixture-
Mixture contains 20% iron by mass, in 50g will be 20%x 50 = 10 grams of iron.
-Calculate %bymass of Fe in FeCl₂
56 / 56+2(35.5) X 100= 44.09% iron by mass
-Calculate how many grams of FeCl₂ will provide 10 grams of Iron
FeCl₂ has 44.09% iron chloride by mass,
There will be 10 g Fe in 10/44.09% =23 grams

Question 12

steps to work out balanced symbol equation

Answer 12

find no. of moles of each substance as whole number ratio using equation: Mass Mr Mol

Question 13

why is an excess of 1 reactant commonly used

what is the limiting reactant, why

The amount of product formed is directly proportional to
WHY

Answer 13

-ensure all of other reactant is used
-reactant that completely used up, limits amount of products
-the amount of limiting reactant, (eg 1/2 the limiting reactant= 1/2 the products formed. Doubling limiting reactant = double the product formed AS LONG AS ITS STILL LIMITING)
-reactant particles=more product particles

Question 14

Calculate Mass of Aluminium Oxide formed when 135g of aluminium is burned in air
5 steps

Answer 14

-write balanced equation- use crossover method and then balance
4Al +3O₂ —–> 2Al₂O₃
-calculate relative formula masses
Al:27 Al₂O₃:102
-Find mols of substance that you know mass of
135g of Al.
Mols= mass/Mr = 135/27 = 5 MOLS
-use balanced equation to work out moles of other substance (use ratios) 4: 2 but we have 5 Mols so…. 5: 2.5
-Use moles and Ar/Mr of other substance to work out its MASS
mass=molsxMr = 2.5x102
= 255 g of aluminium oxide

Question 15

at the same …… and ……. equal numbers of mols of any …… will occupy the same ……..

Answer 15

at the same temperature and pressure equal numbers of moles of any gas will occupy the same volume

Question 16

at room temp and pressure , 1 mole of ANY gas occupies ……
Formula to find vol of known mass of gas
What is room temp + pressure

Answer 16

24 dm³
Volume= (mass of gas / Mr of gas) x 24
room temp=20*c
room pressure= 1 atm

Question 17

How much Carbon Dioxide formed when 30dm³ of Oxygen reacts with carbon Monoxide.
2CO + O₂ —> 2CO₂

Answer 17

1 mol of O₂ = 30dm³
1O₂ : 2CO₂
1:2 ratio
30dm³ : 60dm³
ANSWER= 60dm³

Question 18

what is the relative formula mass of 2KBr
Ar of K=39 Ar of Br=80

Answer 18

39+80=119
DON’T use LARGE NUMBERS to calculate Mr

Question 19

relative atomic mass=

Answer 19

sum of (mass of isotope x isotope abundance)/ 100

Question 20

%mass of an element in a compound=

Answer 20

(Ar x no. of atoms of that element/ Mr of compound) x100

Question 21

concentration=
2 equations

Answer 21

mass of solute(g) /volume of solvent(dm³)
number of moles of solute (mol) /volume of solvent(dm³)

Question 22

number of mols=

Answer 22

mass/relative formula mass

Question 23

mean rate of reaction=

Answer 23

quantity of reactant used/ time
quantity of product formed/time

Question 24

Rf value=

Answer 24

distance moved by substance/distance moved by solvent

Question 25

overall energy change=

Answer 25

energy used to break bonds- energy released when bonds are made

Question 26

percentage yield=

Answer 26

(mass of product made/ maximum theoretical mass of product) x100

Question 27

atom economy

Answer 27

(relative formula mass of desired product/ sum of relative formula masses of all reactant ) x100

Question 28

volume of gas=

Answer 28

number of moles of gas x 24

Question 29

concentration is measured in….
The more solute there is in a volume, the more…
Concentration =
How do u go from cm³ to dm³

Answer 29

mol/dm³ or g/dm³
concentrated the solution
mass OR number of mols of solute/volume of solvent
Divide by 1000

Question 30

atom economy tells us
Why is a high atom economy important

100% atom economy means

Answer 30

the amount of starting material that end up as useful products. - the mass of wasted reactant
for sustainable development and economic reasons
-all atoms in reactants turned into DESIRED products

Question 31

why is low atom economy reactions bad x3

how can this be changed

Answer 31

-makes lots of waste- must be disposed of
-use recources quickly- unsustainable if materials non renewable.
-not profitable- raw materials are costly and waste products costly to RESPONSIBLY remove + dispose of

-should find way to use waste product-There is more than 1 way to make a product so make reaction create useful ‘‘by products’’ instead of ‘‘waste products’’

Question 32

what affects why particular reaction chosen to produce product
5 things

Answer 32

atom economy
yield
rate
equilibrium position
usefulness of by products

Question 33

why is it not possible to always get the % yield that was calculated
x3

what is yield
what is % yield

Answer 33

-reaction may not go to completion bcse its reversible, products can turn back into reactants
-some product may be lost when separated from reaction mixture
-some reactants may react in different ways to expected
YIELD- amount of product obtained
yield compared w/ maximum theoretical yield

Question 34

why might some reactants react in ways differently than expected
x2

Answer 34

react w/ gases in the air
react w/ impurities in the mixture
end up forming extra products

Question 35

-how might some liquid/ solid be lost when separated from reaction mixture

Answer 35

When filtering a liquid to remove solids- u lose abit of liquid/ solid
-if you want to keep the liquid- youll lose the bit that remains w/ the solid + filter paper
-if you want to keep the solid- some will get left behind when scraped of filter paper
-solid/liquid lost when transferred from 1 container to another- left on inside surface of old container