Year 11 Chemistry

Relative Atomic Masses

Mass Spectrometry

Masses

• E.g. What is the average mass of one

smartie?

How much will 200 smarties weigh?

What is the mass of a hydrogen atom?

What is the mass of a hydrogen atom?

• This is not as simple because atoms are tiny and therefore their mass is exceptionally small.

Chemists use relative masses

What is relative mass ?

Mass of a $1 coin = 3 mu

Mass of 30 $1 coin

= 90 mu

What is relative mass ?

A 5c coin is 3 times lighter than a $1 coin so its mass relative to the $1 coin is

1 mu

A 50c coin has a relative mass of 15mu, so it is

5 times heavier than a $1 coin

Mass of a $1 coin = 3 mu

MASSES OF ATOMSAn atoms mass is extremely small.

Eg one atom of carbon has an approximate mass of 2 x 10-23 g.

ie 0.00000000000000000000002g

Chemists don’t use these types of masses

because

• Such small masses cannot be measured

accurately in experimental work and

• are awkward to work with in calculations

RELATIVE MASSESChemists more than 200 years ago used a

relative scale to compare weights of atoms to

each other.

Dalton assigned a H atom a mass of 1.

According to his scale a helium atom has a

relative mass of 4 because it is 4 times as heavy.

RELATIVE MASSES

• Using Dalton’s scale a carbon atom has a relative mass of 12

because a carbon atom is twelve times

heavier than a hydrogen atom

RELATIVE MASSESDalton assigned a magnesium atom a relative

atomic weight of 24.

A Mg atom is 24 times heavier than a H atom

and twice as heavy as a C atom.

Magnesium atomCarbon atom

Comparing Masses

• In 1961 Dalton’s method of comparing masses of atoms was replaced by IUPAC-

• International Union of Applied Physics and Chemistry.

IUPAC RELATIVE MASSESIUPAC decided that the most common isotopeof C which is 12C would be used as a reference standard and assigned an atom of 12C a mass of 12 exactly.

Using this scale the helium isotope is assigned a relative mass of 4

Comparing a helium atom to a carbon atomThe He atom is 3 times lighter

RELATIVE ATOMIC MASSES

A Krpton atom that is given a relative mass of 36

A Kr atom is three times heavier than a 12C atom

RELATIVE ATOMIC MASSESAll isotopes of elements are given a relative

isotopic mass compared to the 12C isotope.

There are 3 isotopes of Mg24Mg25Mg26Mg

These 3 atoms are different because

they have different numbers of neutrons

Abundances of Isotopes

In a sample of pure Mg you will find the isotopes of Mg always occur in the following quantity

78.7% 24Mg10.13% 25Mg11.17% 26Mg

Like Magnesium most elements exist as a mixture of isotopes.

Eg 1H, 2H and 3H

Finding Relative Atomic Masses

Thomson (1913) discovered some elements

had atoms with different masses using an

instrument called a mass spectrometer.

Mass Spectrometer – Principle

• Separates using magnetic attraction and charge.

• If a force is applied at right angles to the path of a moving object, the force will change the object’s direction.

• A lighter object will be deflected more from its original path than a heavier one.

• A more highly charged ion will be deflected more than a one with a lower charge.

A Mass Spectrometer

• http://www.colby.edu/chemistry/OChem/DEMOS/MassSpec.html

1. The element is vaporised

2. Atoms are ionised by knocking one or more electrons off to give a positive ion. Positive ions are accelerated to high speeds by a magnetic field so that they all have the same kinetic energy.

3.The ions are then deflected by a magnetic field according to their masses.

The lighter they are, the more they are deflected.

The amount of deflection also depends on charge on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.

4. The collector measures the current due to the different ions and the data is recorded as a mass spectrum

is used to measure relative isotopic masses.

Relative height of peak = relative abundance

Position of peak on x axis = relative isotopic mass

Mass Spectrometer

This element has 2 isotopes.

The lightest isotope has a relative atomic mass of 35 & an abundance of 75%.

The heavier isotope has a relative atomic mass of 37 & an abundance of 25%.

Mass Spec of an element

Summing Up

Relative masses of isotopes of an element are

determined by an instrument called a mass

spectrometer

This separates isotopes and determines their

mass relative to the 12C isotope

and gives you the relative abundance of the

isotopes on a graph called a mass spectrum.

http://www.colby.edu/chemistry/OChem/DEMOS/MassSpec.html

Mass Spectrum of MagnesiumEach peak represents a different isotope.

The position of each peak on the horizontal axis indicates the relative isotopic mass which tells us how heavy the atoms of each isotope is compared to the12C isotope.

The relative heights of the peaks correspond to the relative abundance of the isotopes.

AVERAGE RELATIVE ATOMIC MASSESDon’t normally worry about the isotopes of anelement because they always occur in the same proportions and behave identically in chemical reactions. Chemists use what is known as average relativeatomic masses

This is an average mass of all the isotopes of anelement compared to12C and it is given the symbol

Ar.

AVERAGE RELATIVE ATOMIC MASSES

Ar(Ti) = 47.90

A Ti atom on average is about 4 times heavier than a C atom. (47.9 ÷ 12)

CALCULATING Ar

Calculate the average relative atomic mass of Magnesium given:Isotope Relative Mass Abundance

24Mg 23.99 78.7%25Mg 24.89 10.13 %26Mg 25.98 11.17%

Assume we have 100 atoms of Mg. mass contributed by the 24Mg isotope is 23.99 x 78.7mass contributed by the 25Mg isotope is 24.89 x 10.13mass contributed by the 26Mg isotope is 25.98 x 11.17

Total mass of 100 Mg atoms = 23.99 x 78.7 + 24.89 x 10.13 + 25.98

Finding ArTotal mass of 100 Mg atoms = 23.99 x 78.7 + 24.89 x 10.13 + 25.98 x 11.17

Ar(Mg) = 23.99 x 78.7 + 24.89 x 10.13 + 25.98 x 11.17 100

Ar(Mg) = 24.3

This is not the true mass of a Mg atom but its relative mass compared to a 12C atom.

Finding Ar

The general rule is:

Ar = Σ(relative isotopic mass x abundance)

100

Finding Ar

• Find the relative atomic mass of Chorine.

Isotope Relative Mass Abundance35Cl 34.969 75.80%37Cl 36.966 24.20%

Ar(Cl) = 34.969 x 75.8 + 36.966 x 24.2 100

Ar(Cl) = 35.45

Find Ar(O)Isotopes Relative Isotopic Mass

Abundance16O 15.995 99.7617O 16.999 0.0418O 17.999 0.20

Ar(O) = 15.995 x 99.76 + 16.999 x 0.04 + 17.999 x 0.2

100

Ar(O) = 16

Calculating Abundances

• The relative atomic mass of Rubidium is 85.47. The relative masses of the two isotopes are 84.94 and 86.94.

• Calculate the relative abundances of both isotopes.

Calculating AbundancesRelative mass lightest isotope = 84.94

Relative mass heaviest isotope = 86.94

Ar = 85.47

Abundance of lightest isotope = x

Abundance of heaviest isotope = 100 – x

Ar = ∑(relative isotopic mass x abundance)

100

85.47 = 84.94 × x + 86.94(100 – x)

100

Calculating Abundances85.47 = 84.94 x x + 86.94(100 – x)

100

8547 = 84.94x + 8694 – 86.94x

-147 = -2x

x = 73.5

Abundance of lightest isotope = 73.5%

Abundance of heaviest isotope = 26.5%

Relative Atomic Masses

• Can be read from the Periodic table or a table of relative atomic masses.

Relative Molecular and Formula Mass

We can also find out how heavy a molecule

of a compound is.

Mr – relative molecular mass or formula mass

Find Mr of H2O

To find Mr simply add the relative atomic

masses of each atom in the molecule.

Mr(H2O) = 2 x Ar(H) + Ar(O)

= 2 x 1.008 + 15.999

= 18

A water molecule is 1.5 times heavier than a

carbon atom. (18 ÷ 12)

Find Mr of C6H12O6

Mr(C6H12O6) = 6 x Ar(C) + 12 x Ar(H) + 6 x Ar(O)

= 6 x 12 + 12 x 1 + 6 x 16

= 180

A glucose molecule is 15 times heavier than a

carbon atom. (180 ÷ 12)

glucose

Ionic Compounds

Eg NaCl

For compounds that don’t consist of

molecules we find the formula mass.

Mr(NaCl) = 23 + 35.5

= 58.5

Course Work

• Read Chapter 2 pp 37 – 38

Chapter Review Questions pg 49 Q 6, 7, 8,

• Chapter Questions pg 39 Q 10, 11, 14b, 15, 16, 18

• Complete mass spec worksheet