1.3 Chemical Calculations

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WJEC A-Level Chemistry

40 Terms

1

What is the relative isotopic mass?

The mass of an atom of an isotope relative to one-twelfth the mass of one atom of carbon-12.

e.g. 35Cl has a relative isotopic mass of 35

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2

What is the relative atomic mass (Ar)?

The average mass of one atom of the element relative to one-twelfth the mass of one atom of carbon-12.

  • found on a periodic table

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3

What is the relative formula mass (Mr)?

The average mass of a molecule relative to one-twelfth the mass of one atom of carbon-12.

  • add up all the Ar

e.g. CaCO3 40.1 + 12 + (16×3) = 100.1

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4

What is a mole?

One mole is the amount of substance that contains the same number of particles as there are atoms of carbon in exactly 12g of carbon-12.

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5

Mass =

Mr x moles

<p>Mr x moles</p>
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6

What is the molar mass?

One mole of any substance has a mass equivalent to the Mr of that substance in grams (gmol-1).

e.g. MgCl2 - Mr=95.3, molar mass = 95.3 gmol-1

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7

What is the empirical formula?

Simplest formula of a substance

  1. symbol of elements

  2. % or mass

  3. Ar

  4. moles

  5. divide by smallest mole

  6. ratio

<p>Simplest formula of a substance</p><ol><li><p>symbol of elements</p></li><li><p>% or mass</p></li><li><p>Ar</p></li><li><p>moles</p></li><li><p>divide by smallest mole</p></li><li><p>ratio</p></li></ol><p></p>
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8

How to calculate reacting masses?

  1. balanced equation

  2. moles of one

  3. ratio for moles of what we need

  4. mass=mrxmoles

<ol><li><p>balanced equation</p></li><li><p>moles of one</p></li><li><p>ratio for moles of what we need</p></li><li><p>mass=mrxmoles</p></li></ol><p></p>
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9

Percentage yield =

mass of product obtained / maximum theoretical mass x 100

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10

Why will percentage yield always be less than 100%?

Due to loss of product during transfer,separation or an incomplete reaction

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11

Atom economy =

mass of required product / total mass of reactants x 100

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12

Concentration (moldm-3) =

moles / volume (dm3)

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13

cm3 to dm3

/1000

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14

What is neutralisation?

  • When the number of moles of H+ ions from the acid is equal to the number of OH- ions form the alkali

  • If an acid and alkali have different concentrations then a different volume of each is needed

  1. moles

  2. ratio

  3. concentration/volume

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15

What is molar volume?

The volume of one mole of gas is always the same whatever the gas as long as the temperature and pressure are constant

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16

How much volume does 1 mole of gas occupy at standard temperature and pressure (stp 0°C)

22.4dm3

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17

How much volume does 1 mole of gas occupy at room temperature and pressure (rtp 20°C)

24.0dm3

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18

molar volume =

volume / moles

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19

What is the ideal gas equation?

knowt flashcard image
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20

What is the gas equation under changing conditions?

knowt flashcard image
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21

Percentage error =

maximum error / quantity measured x 100

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22

What happens to percentage error on burretes or for temperature change?

Must be doubled

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23

What is a mass spectrometer?

an instrument to accurately determine the relative atomic/isotopic/formula mass.

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24

What does a mass spectrometer measure?

  • mass of each different isotope of the element

  • relative abundance of each isotope of the element

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25

What are the stages of the mass spectrometer?

  1. vaporisation

  2. ionisation

  3. acceleration

  4. deflection

  5. detection

<ol><li><p>vaporisation</p></li><li><p>ionisation</p></li><li><p>acceleration</p></li><li><p>deflection</p></li><li><p>detection</p></li></ol><p></p>
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26

What happens during vaporisation stage of mass spectrometer?

liquid sample is vaporised into a gas

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27

What happens during ionisation stage of mass spectrometer?

beam of electrons knock electrons from atoms or molecules in sample, forming positive ions

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28

What happens during acceleration stage of mass spectrometer?

ions accelerated so all ions have same kinetic energy

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29

What happens during deflection stage of mass spectrometer?

deflected according to mass and charge

  • heaver ions are deflected less than light ones

  • 2+ ions are deflected twice as much as 1+ ions

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30

What happens during detection stage of mass spectrometer?

  • on striking the detector ions accept electrons, lose their charge and create a current

  • current created is proportional to the abundance of each ion

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31

What are the uses of mass spectrometry?

  • identifying trace compounds in forensic science

  • identifying unknown compounds e.g. banned substances in athletes

  • analysing molecules in space

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32

What are the conditions of mass spectrometry?

vacuum so air molecules don’t interfere with moving ions

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33

Mass spectra of elements

knowt flashcard image
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34

Double charged ions

  • greater charge on ion means more deflection

  • double charge = half mass

  • smaller abundance as double ionisation is rare

<ul><li><p>greater charge on ion means more deflection</p></li><li><p>double charge = half mass</p></li><li><p>smaller abundance as double ionisation is rare</p></li></ul><p></p>
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35

Where are the peaks on the mass spectra of chlorine molecule (Cl2)

  • at m/z 70 due to 35Cl — 35Cl

  • at m/z 72 due to 35Cl — 37Cl and 37Cl — 35Cl

  • at m/z 74 due to 37Cl — 37Cl

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36

Probability of peaks on the mass spectra of chlorine molecule (Cl2)

  • 70 - ¾ x ¾ = 9/16

  • 72 - ¾ x ¼ + ¼ x ¾ = 6/16

  • 74 - ¼ x ¼ = 1/16

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37

What is a standard solution used for?

titration to find concentration of another solution

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38

How to make a standard solution?

  1. transfer all solid into a beaker, add water until it all dissolves

  2. pour into a volumetric flask through a funnel

  3. fill rest of flask to graduation mark with water

  4. add stopper to flask and shake vigorously

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39

Standardisation of an acid solution

Na2CO3 + 2HCl —> 2NaCl + CO2 + H2O

  • Na2CO3 = standard solution

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40

Titration calculations

  • concordant results = +— 0.20cm3

  • burette reading = +— 0.05cm3

  • don’t include rough titration when calculating mean titre

<ul><li><p>concordant results = +— 0.20cm<sup>3</sup></p></li><li><p>burette reading = +— 0.05cm<sup>3</sup></p></li><li><p>don’t include rough titration when calculating mean titre</p></li></ul><p></p>
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