Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 1

Two Isotopes of Carbon

C-12 has 6 protons and 6 neutrons with an abundance of 98.90%

C-13 has 6 protons and 7 neutrons with an abundance of only 1.10%

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 2

The Mole

Exactly 12 g of carbon-12

The number of units (atoms) in this amount of carbon-12 is our “bunch”. This bunch is called a mole (symbolised by mol)

The number of atoms in one mol of carbon-12 is called the Avogadro's Number and it is symbolised by NA

236.02214129 10AN

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 3

Relationship between Mass, Number of Moles and Number of Atoms

Example: How many atoms are there in 0.586 g of potassium?

Let  𝑁=number   of   atoms  of   K   in   the   sample .   It   is  given  by

𝑁=0.586  g   K×1   mol

39.10   g×

6.02214129×1023atoms1  mol

=9.03×1021   atoms  (3 SF )

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 4

Mass Spectrometer

Gas is ionised by the electron beam. They are accelerated by the plates. Depending on the charge/mass ratio, each group of ions follows a separate route before they strike the screen at different points.

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 5

Mass Spectrum of Neon

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 6

Empirical Formula

C H O

42.1% 6.48% 51.4%

Then divide each atom by its atomic mass42.1/12.0107=3.5 6.48/1.00794=6.42 51.4/15.9994=3.21

This means we have this ratio of atoms in the molecule. Since we cannot have fractional atoms, we have to normalise this ratio

We have to find a multiplier to make all values an integer. In this case it is 11

Therefore the empirical formula is C12H22O11

Then divide each value by the smallest value3.5/3.21=1.0903 6.42/3.21=2 3.21/3.21=1

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 7

Flowchart for Empirical Formula Determination

Intelligent use of the flowchart will enable empirical formula determination.

MAKE SURE THAT THE MULTIPLER MAKES SENSE. DO NOT ASSUME A SIMPLE ONE RIGHT AWAY.

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 8

Characterisation of a New Compound

Three independent experiments to complete chemical characterisation of the structure of a new compound

i) Elemental analysis – This tells us what elements we have in the compound – Qualitative analysisii) Determination of percentages of the elements – This tells us how much we have of each element in the compound – Quantitative analysis – Empirical formula iii) Determination of the molecular mass – Molecular formula

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 9

Apparatus for Percentage Determination

Ethanol, an organic alcohol, is burned in a high-temperature oven in presence of a lot of oxygen to give off water vapour and carbon dioxide. The products are absorbed and their mass are determined. Hence percentage of carbon and hydrogen can be determined. Rest is, of course, oxygen.

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 10

Writing Chemical Equations

1. One molecule of nitrogen is needed to react with 3 molecules of hydrogen, yielding 2 molecules of ammonia

2. One mole of nitrogen is needed to react with 3 moles of hydrogen, yielding 2 moles of ammonia

3. 28.0 g of nitrogen are needed to react with 3 x 2.02 g of hydrogen, yielding 2 x 17.03 g of ammonia

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 11

Flowchart for Stoichiometric Calculations

 1. Write a complete and balanced equation2. Convert given masses of reactants to moles3. Use the coefficients to write down mole ratios4. Using mole ratios, calculate moles of the product5. Change moles of product into mass of product (if required)  

Mass Relationships in Chemical Reactions

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 12

2NO + O2 2NO2

NO is the limiting reagent

O2 is the excess reagent

NO is used up first in the reaction.

Limiting Reagent – Excess Reagent