Mass Relationships in Chemical Reactions General Chemistry I CHM 111 Dr Erdal OnurhanSlide 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 OnurhanSlide 2 The Mole 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 N A

Mass Relationships in Chemical Reactions General Chemistry I CHM 111 Dr Erdal OnurhanSlide 3 Relationship between Mass, Number of Moles and Number of Atoms Example: How many atoms are there in g of potassium?

Mass Relationships in Chemical Reactions General Chemistry I CHM 111 Dr Erdal OnurhanSlide 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 OnurhanSlide 5 Mass Spectrum of Neon

Mass Relationships in Chemical Reactions General Chemistry I CHM 111 Dr Erdal OnurhanSlide 6 Empirical Formula CHOCHO 42.1%6.48%51.4% Then divide each atom by its atomic mass 42.1/ = / = / =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 C 12 H 22 O 11 Then divide each value by the smallest value 3.5/3.21= /3.21=2 3.21/3.21=1

Mass Relationships in Chemical Reactions General Chemistry I CHM 111 Dr Erdal OnurhanSlide 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 OnurhanSlide 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 analysis ii) 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 OnurhanSlide 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 OnurhanSlide 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 g of nitrogen are needed to react with 3 x 2.02 g of hydrogen, yielding 2 x g of ammonia

Mass Relationships in Chemical Reactions General Chemistry I CHM 111 Dr Erdal OnurhanSlide 11 Flowchart for Stoichiometric Calculations 1. Write a complete and balanced equation 2. Convert given masses of reactants to moles 3. Use the coefficients to write down mole ratios 4. Using mole ratios, calculate moles of the product 5. Change moles of product into mass of product (if required)

Mass Relationships in Chemical Reactions General Chemistry I CHM 111 Dr Erdal OnurhanSlide 12 2NO + O 2 2NO 2 NO is the limiting reagent O 2 is the excess reagent NO is used up first in the reaction. Limiting Reagent – Excess Reagent