Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Atoms, molecules and
stoichiometry
AS

Atoms
Atoms are tiny invisible particles
of an element that cannot be
created or destroyed
...

Examples : Chlorine , Carbon dioxide

Units
•
•
•
•
•
•
•

1 kg = 1000 g
1 km = 1000 m
1 m = 100 cm
1 cm = 10 mm
1 m3 = 106 cm3 / 1 cm3 = 10-6 m3
1 L = 1000 mL = 1000 cm3 (1 mL = 1 cm3)
1 L = 1 dm3 = 10-3 m3

Isotopes
Isotopes are atoms of same element that have the
similar proton number but different nucleon number
...


Relative atomic mass is the weighted average mass
of naturally occurring atoms of an element on a scale
where an atom of carbon-12 has a mass exactly 12
units
...
This is calculated in the same way as for
relative molecular mass
...
Calculate the relative formula mass, Mr of
1
...
(NH4)2SO4

3
...
5H20
4
...
7H2O

Avogadro's Number and
Moles
Avogadro constant, L is defined
as the number of carbon atoms
in exactly 12 g of carbon-12
which is 6
...

Mole is defined as the amount of substance containing
Avogadro’s number of particles of that substance
...
02 x 1023 atoms
1 mole of H20 molecules = 6
...
02 x 1023 ions

Questions:
1
...
How many hydrogen atoms are there in 1 mole of H2O
molecules?
3
...
How many SO42- ion and oxygen atom in 1 mole CuSO4 ?

How to obtain an accurate relative atomic masses?
 Mass Spectrometer

A mass spectrometer can be used to measure the mass
of each isotope present in an element
...
It shows where the ion appears and how many ion
appears
The mass spectrum of an element provides the following
information:
1
...


2
...


3
...

The relative atomic mass, Ar, of an element is the weighted
average of the isotopic masses according to their relative
abundance
...
Calculate the
RAM for Cl

Example 2

What is the relative atomic mass of the element?

Example 3

Calculate relative atomic mass the naturally occurring
silicon based on table above:

Q:
The diagram shows the mass spectrum of a sample of zinc
...


A
...
65
...

D
...
25
65
...
What is the mass of 0
...
How many moles is 54 g of water, H2O?

3
...
7H20 in 1
mole ?
4
...
How many atoms are there in 11 g of propane , C3H8 ?

If you write a formula for a substance in a
calculation, it is often convenient to take that
formula as meaning 1 mole of that substance
...

Example 1 :
What mass of calcium oxide could be obtained
by heating 25 g limestone?

Solution

CaCO3  CaO + CO2
1mole of CaCO3 produces 1 mol of CaO
100 g CaCO3 produces 56 g of CaO

Do the simple proportion sum:
100 g CaCO3 gives 56 g of CaO
25 g of CaCO3 gives 25/100 x 56 g = 14 g

Question
1
...

a
...
What mass of titanium is produced from 380 g of titanium
chloride ? Give your answer to 3 s
...
What mass of sodium is required to produce 1 tonne of
titanium?
2
...
67 g of aluminium chloride was dissolved in water and an
excess of silver nitrate solution was added to give ppt of silver
chloride
...

Q1: Titanium (IV) oxide, TiO2, is brilliantly white and much of the oxide
produced is used in the manufacture of paint
...
0tonnes of the ore
ilmenite, FeTiO3?
A
...
0tonnes
B
...
7tonnes
C
...
0tonnes
D
...
7tonnes

• [AS Nov 2008 Paper I Q1]

Use of the Data Booklet is relevant to this question
...
How many molecules of an amino acid
(Mr = 200) would this be?
A
...
200
C
...
360 000

• [AS Nov 2003 Paper I Q1]

Use of the Data Booklet is relevant to this question
...
216g
...
8
...
1
...
1
...
4
...

The coefficient in the balanced equation indicates the
relative amount (measured in moles) of the reactants
and products
...


Limiting reagents are those completely reacted at the
end of the reaction and it determines the yield of the
product
...

Actual yield may be less due to incomplete reaction or
product loss
...
Given that,
C7H6O3 + CH3OH → C8H8O3 + H2O

Salicylic acid –
treatment for acne

We can use the formula of a compound and relative
atomic masses to calculate the percentage by mass
of a particular element in the compound
...


Calculate the percentage by mass of iron in iron (III)
oxide, Fe2O3
...

Calculate the percentage by mass of caebon in
ethanol, C2H5OH
...
Ans: % mass of iron = 69
...
Ans: % mass of C = 26
...
Change percentage composition to mass, m ( in g)

2
...

3
...


Example 1

Calculate the empirical formula of a compound that
has the composition: 12
...
1% hydrogen
and 85
...


Example 2:

When 1
...
55
g of an oxide of phosphorus is produced
...
Its relative
molecular mass is 187
...
Deduce the molecular formula of
this compound ?

Balancing Ionic Equations
When ionic compounds dissolve in the water, the ions
separate from each other
...
What is its concentration in mol dm-3?

Example 2:

What is the concentration of a 0
...
10 g of sodium hydrogen carbonate, NaHCO3
in 250 cm3 of solution ?

Example 4

What mass of barium sulfate would be produced by adding
excess barium chloride solution to 20
...
100 mol dm -3 ?

Example 5

10 cm3 of a 3 moldm-3 sulfuric acid is diluted with water
to give a volume of 250 cm3, what is the concentration
of the diluted solution?

Calculating solution concentration and deducing
stoichiometry by titration
...
In order to do this, we need to know the
concentrations and the volumes of both the reactants
...
5 mol dm-3
HCl require 20 cm3 of NaOH for
complete neutralisation
...


Example 2

Solution:

Question 1

Questions
1) The equation for the reaction of strontium hydroxide and
hydrochloric acid is shown below
Sr(OH)2 + 2HCl  SrCl2 + 2H2O
25cm3 of a solution of strontium hydroxide was exactly neutralised by
15
...
1 mol dm-3 hydrochloric acid
...

2) 25
...
05mol dm-3 solution of a metal hydroxide was titrated
against a solution of 0
...
It required 12
...

Deduce the stoichiometry of this reaction
...
It can be disposed of safely by
reaction with sodium hydroxide
...
5mol dm-3 NaOH (aq) needed to
dispose of 0
...
8cm3
B
...
5cm3
C
...
80cm3

• [AS June 2006 Paper I Q1]

Use of the Data Booklet is relevant to this question
...
The recommended usage of fertiliser is 14g of
fertiliser per 5dm3 of water
...
0
...
0
...
0
...
0
...
75g dm-3 of hydrated sodium
carbonate, Na2CO3
...

• FC2 is 0
...

25
...
The titre
value obtained is 23
...

(a) Calculate how many moles of the acid were run from the burette
into the conical flask during the titration of FC1 with FC2
...
0cm3 of FC1
...


(d) Calculate the mass of anhydrous sodium carbonate present in
1
...

• [Ar: Na, 23
...
0; O, 16
...
]
(e) Calculate the mass of water present in the hydrated sodium
carbonate
...
A gaseous hydrocarbon, CxHy, explodes with excess O2
according to the general equation
...
40 cm3 of CO2 were
produced as well as 60 cm3 of steam
...

Write the equation for the reaction and so find the formula
of the hydrocarbon

Example 2:
10 cm3 of an unknown hydrocarbon, CxHy, required 30 cm3
of oxygen for complete combustion
...
Write the equation for the reaction and so find the
formula of the hydrocarbon
...
A spark
was passed through the mixture, and the resulting gases
were allowed to cool back to room temperature
...
When this gas was exposed to sodium
hydroxide solution, the volume fell to 40 cm3
...

• CS2 (g) + 3O2 (g)  CO2 (g) + 2SO2 (g)
A sample of 10cm3 of carbon disulphide was burned in 50cm3 of
oxygen
...
All measurements were made at
the same temperature and pressure, under such conditions that
carbon disulphide was gaseous
...
200mol of a hydrocarbon undergo complete combustion to
give 35
...
4g of water as the only
products
...
C2H4
B
...
C4H4
D
...
When 10cm3 of the hydrocarbon is burned in 70cm3 of
oxygen (an excess), the final gaseous mixture contains 30cm3 of
carbon dioxide and 20cm3 unreacted oxygen
...
What is the
formula of the hydrocarbon?
A
...
C3H6
C
...
C4H10
• [AS June 2005 Paper I Q1]

Q4: A mixture of 10cm3 of methane and 10cm3 of ethane are
sparked with an excess of oxygen
...
What volume of gas is absorbed by the
alkali?
A
...
20cm3
C
...
40cm3
• [AS Nov 2002 Paper 1 Q1]



---