calculations involving formulae national 5 chemistry knockhardy publishing

CALCULATIONS INVOLVING CALCULATIONS INVOLVING FORMULAEFORMULAENational 5 ChemistryNational 5 Chemistry

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

CALCULATIONS INVOLVING FORMULAECALCULATIONS INVOLVING FORMULAE

INTRODUCTION

This Powerpoint show is one of several produced to help students understand selected GCSE Chemistry topics. It is based on the requirements of the SQA specification.

Individual students may use the material at home for revision purposes and it can also prove useful for classroom teaching with an interactive white board.

Accompanying notes on this, and the full range of Chemistry topics, are available from the KNOCKHARDY WEBSITE at...

www.knockhardy.org.uk

All diagrams, photographs and any animations in this Powerpoint are original and created by Jonathan Hopton. Permission must be obtained for their use in any work that is distributed for financial gain.

All diagrams, photographs and any animations in this Powerpoint are original and created by Jonathan Hopton. Permission must be obtained for their use in any work that is distributed for financial gain.

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CONTENTSCONTENTS

• Working out simple formulae

• Relative formula mass

• Percentage composition

• Reacting masses

• Yield and percentage yield

CALCULATIONS INVOLVING FORMULAECALCULATIONS INVOLVING FORMULAE

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chlorine gas Cl2

hydrogen gas H2

nitrogen gas N2

oxygen gas O2

hydrochloric acid HClnitric acid HNO3

sulphuric acid H2SO4

sodium hydroxide NaOHsodium chloride NaClsodium carbonate Na2CO3

potassium chloride KClmagnesium oxide MgOmagnesium hydroxide Mg(OH)2

magnesium carbonate MgCO3

magnesium sulphate MgSO4

calcium carbonate CaCO3

calcium chloride CaCl2

SOME COMMON FORMULAESOME COMMON FORMULAE

Many formulae can be worked out by reference to a table of ions.

Others just have to be known off by heart.

Many formulae can be worked out by reference to a table of ions.

Others just have to be known off by heart.

hydrogen H+ chloride Cl¯sodium Na+ bromide Br¯potassium K+ iodide I¯lithium Li+ hydroxide OH¯rubidium Rb+ nitrate NO3¯

caesium Cs+ nitrite NO2¯

silver(I) Ag+ hydrogencarbonate HCO3¯

ammonium NH4+ hydrogensulphate HSO4¯

calcium Ca2+ sulphate SO42-

barium Ba2+ sulphite SO32-

magnesium Mg2+ sulphide S2-

zinc Zn2+ oxide O2-

iron(II) “ferrous” Fe2+ carbonate CO32-

copper(II) Cu2+

aluminium Al3+

iron(III) “ferric” Fe3+

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TABLE OF IONSTABLE OF IONS








zinc Zn2+ oxide O2-


copper(II) Cu2+

aluminium Al3+


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NUMBER OF ELECTRONS IN OUTER SHELL OF ATOMS

1 2 3 6 7








zinc Zn2+ oxide O2-


copper(II) Cu2+

aluminium Al3+


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Formulae can be worked out by balancing the charges so that…

negative charge = positive charge

Formulae can be worked out by balancing the charges so that…


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CONSTRUCTING FORMULAECONSTRUCTING FORMULAE

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Many formulae can be worked out by balancing the charges so that…


Q.1 Write out the correct formula for each of the following compounds.

a) sodium chlorideb) magnesium sulphatec) potassium oxided) calcium chloridee) copper(II) nitrate

sodium ion Na+

chloride ion Cl¯

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sodium ion Na+

chloride ion Cl¯

Na+ Cl¯

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sodium ion Na+

chloride ion Cl¯

Na+ Cl¯

ONE Na+ balances ONE Cl¯

FORMULA is NaCl

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a) sodium chloride NaClb) magnesium sulphatec) potassium oxided) calcium chloridee) copper(II) nitrate

magnesium ion Mg2+

sulphate ion SO42-

Mg2+ SO42-

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a) sodium chloride NaClb) magnesium sulphatec) potassium oxided) calcium chloridee) copper(II) nitrate

magnesium ion Mg2+

sulphate ion SO42-

Mg2+ SO42-

ONE Mg2+ balances ONE SO42-

FORMULA is MgSO4

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a) sodium chloride NaClb) magnesium sulphate MgSO4

c) potassium oxided) calcium chloridee) copper(II) nitrate

potassium ion K+

oxide ion O2-

K+ O2-

K+

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c) potassium oxided) calcium chloridee) copper(II) nitrate

potassium ion K+

oxide ion O2-

K+ O2-

TWO K+ balance ONE O2-

FORMULA is K2O

K+

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c) potassium oxide K2O

d) calcium chloridee) copper(II) nitrate

calcium ion Ca2+

chloride ion Cl¯Ca2+ Cl¯

Cl¯

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d) calcium chloridee) copper(II) nitrate

calcium ion Ca2+

chloride ion Cl¯Ca2+ Cl¯

ONE Ca2+ balances TWO Cl¯

FORMULA is CaCl2

Cl¯

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d) calcium chloride CaCl2e) copper(II) nitrate

copper(II) ion Cu2+

nitrate ion NO3¯

Cu2+ NO3¯

NO3¯

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copper(II) ion Cu2+

nitrate ion NO3¯

Cu2+ NO3¯

ONE Cu2+ balances TWO NO3¯

FORMULA is Cu(NO3)2

NO3¯

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copper(II) ion Cu2+

nitrate ion NO3¯

Cu2+ NO3¯

ONE Cu2+ balances TWO NO3¯

FORMULA is Cu(NO3)2

NO3¯

NOTE THE BRACKETSIf the brackets weren’t there, the formula would appear as CuNO32 (thirty two oxygens). Brackets are used when the ion has more than one element in it.

eg NO3¯ OH¯ NH4+

NOTE THE BRACKETSIf the brackets weren’t there, the formula would appear as CuNO32 (thirty two oxygens). Brackets are used when the ion has more than one element in it.

eg NO3¯ OH¯ NH4+

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Write out the correct formula for each of the following compounds.

a) lithium chloride LiClb) calcium sulphate CaSO4

c) sodium oxide Na2O

d) calcium chloride CaCl2e) iron(II) nitrate Fe(NO3)2

f) potassium sulphate K2SO4

g) calcium hydroxide Ca(OH)2

h) zinc carbonate ZnCO3

i) aluminium oxide Al2O3

j) aluminium sulphate Al2(SO4)3

k) ammonium nitrate NH4NO3

l) ammonium chloride NH4Cl

m) ammonium sulphate (NH4)2SO4

n) iron(III) chloride FeCl3

FORMULAE – Test questionsFORMULAE – Test questions

Write out the correct formula for each of the following compounds.

a) lithium chloride LiClb) calcium sulphate CaSO4

c) sodium oxide Na2O

d) calcium chloride CaCl2e) iron(II) nitrate Fe(NO3)2

f) potassium sulphate K2SO4

g) calcium hydroxide Ca(OH)2

h) zinc carbonate ZnCO3

i) aluminium oxide Al2O3

j) aluminium sulphate Al2(SO4)3

k) ammonium nitrate NH4NO3

l) ammonium chloride NH4Cl

m) ammonium sulphate (NH4)2SO4

n) iron(III) chloride FeCl3HOPTON

FORMULAE – Test questionsFORMULAE – Test questions

RELATIVE FORMULA MASSRELATIVE FORMULA MASS

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The relative formula mass is the sum of the relative atomic masses of ALL the atoms present in the formula


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To calculate this value, it is important that one understands how to work out the number of each type of atom present.



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A small subscript number tells you how many there of the atom just before the subscript





H2O 2 hydrogen atoms

1 oxygen atom (a 1 after the O is unnecessary)



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H2O 2 hydrogen atoms

1 oxygen atom (a 1 after the O is unnecessary)

Ca(OH)2 1 calcium atom

2 lots of everything in the bracketie 2 oxygen atoms and 2 hydrogen atoms

(groups of atoms occurring in brackets include… SO4 NO3 NH4 OH)



a) H2O 2 x H 1 x O

b) NaOH 1 x Na 1 x O 1 x H

c) (NH4)2SO4 2 x N 4 x O 8 x H 1 x S

d) Ca(OH)2 1 x Ca 2 x O 2 x H

e) H2SO4 2 x H 4 x O 1 x S

f) CuSO4.5H2O 1 x Cu 10 x H 9 x O 1 x S


HOW MANY ATOMS OF EACH TYPE ARE IN THE FOLLOWING?

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a) H2O 2 x H 1 x O


c) (NH4)2SO4 2 x N 4 x O 8 x H 1 x S


e) H2SO4 2 x H 4 x O 1 x S


A NUMBER IN THE FRONT OF A FORMULAMULTIPLIES EVERYTHING IN THAT FORMULA



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a) H2O 2 x H 1 x O


c) (NH4)2SO4 2 x N 4 x O 8 x H 1 x S


e) H2SO4 2 x H 4 x O 1 x S



g) 2 NaOH 2 x Na 2 x O 2 x H

h) 3 Ca(OH)2 3 x Ca 6 x O 6 x H

i) 2 Na2HPO4 4 x Na 8 x O 2 x H 2 x P



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a) H2O 2 x H 1 x O


c) (NH4)2SO4 2 x N 4 x O 8 x H 1 x S


e) H2SO4 2 x H 4 x O 1 x S



g) 2 NaOH 2 x Na 2 x O 2 x H

h) 3 Ca(OH)2 3 x Ca 6 x O 6 x H

i) 2 Na2HPO4 4 x Na 8 x O 2 x H 2 x P




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H = 1; C = 12; N = 14; O = 16; Na = 23; Mg = 24; Al = 27;S = 32; Cl = 35.5; K = 39; Ca = 40; Fe = 56; Cu = 64


Water H2O


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Water H2O H + H + O


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Water H2O H + H + O 1 + 1 + 16 = 18


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Water H2O H + H + O 1 + 1 + 16 = 18

Copper CuSO4

sulphate


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Water H2O H + H + O 1 + 1 + 16 = 18

Copper CuSO4 Cu + S + 4 x Osulphate


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Water H2O H + H + O 1 + 1 + 16 = 18

Copper CuSO4 Cu + S + 4 x Osulphate 64 + 32 + 4 x 16


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Water H2O H + H + O 1 + 1 + 16 = 18

Copper CuSO4 Cu + S + 4 x Osulphate 64 + 32 + 4 x 16 = 160


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Water H2O H + H + O 1 + 1 + 16 = 18


TAKE CARE WHEN THERE ARE BRACKETS


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Water H2O H + H + O 1 + 1 + 16 = 18



Calcium Ca(OH)2

hydroxide


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Water H2O H + H + O 1 + 1 + 16 = 18



Calcium Ca(OH)2 Ca + 2 lots of OHhydroxide


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Water H2O H + H + O 1 + 1 + 16 = 18



Calcium Ca(OH)2 Ca + 2 lots of OHhydroxide Ca + O + H + O + H


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Water H2O H + H + O 1 + 1 + 16 = 18




40 16 + 1 + 16 + 1


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Water H2O H + H + O 1 + 1 + 16 = 18




40 16 + 1 + 16 + 1 = 74


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Use the list of relative atomic masses to calculate the relative formula mass of the following compounds

H2 O2 H2O

NaOH CH4 C5H12

NaCl H2SO4 Na2SO4

CaCl2 CaCO3 NH4NO3

CuSO4 Al2O3 Ca(OH)2

Al2(SO4)3H = 1; C = 12; N = 14; O = 16; Na = 23; Mg = 24; Al = 27;S = 32; Cl = 35.5; K = 39; Ca = 40; Fe = 56; Cu = 64


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Use the list of relative atomic masses to calculate the relative formula mass of the following compounds

H2 O2 H2O

NaOH CH4 C5H12

NaCl H2SO4 Na2SO4

CaCl2 CaCO3 NH4NO3

CuSO4 Al2O3 Ca(OH)2

Al2(SO4)3H = 1; C = 12; N = 14; O = 16; Na = 23; Mg = 24; Al = 27;S = 32; Cl = 35.5; K = 39; Ca = 40; Fe = 56; Cu = 64

2 32 18

40 16 72

36.5 98 142

111 100 80

160 102 74

342

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PERCENTAGE COMPOSITIONPERCENTAGE COMPOSITION


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It can be quite useful to know how much of an element is present in a compound.

This is not how many atoms there are BUT what mass is present.

This has some relevance when considering how much metal is in a particular ore.

Metal ores do not just contain a metal, there are other elements present.eg Fe2O3, TiO2, ZnS.


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To calculate the PERCENTAGE BY MASS of a metal in an ore;


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1 Calculate the formula mass (Fe2O3) 56 + 56 + 16 + 16 + 16 = 160


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2 Calculate how much of this is Fe 56 + 56 = 112


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3 Calculate the fraction that is Fe 112160


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4 Calculate the percentage that is Fe 112 x 100 = 160


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4 Calculate the percentage that is Fe 112 x 100 = 70.0%

160


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4 Calculate the percentage that is Fe 112 x 100 = 70.0%

160

Calculate the percentage of each metal in… Al2O3 TiO2

CuFeS2 ZnS NaCl


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REACTING MASSESREACTING MASSES

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The masses of all the atoms at the end of a reaction (THE PRODUCTS) must equal the masses of all the atoms at the start of the reaction (THE REACTANTS).


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copper + sulphuric acid copper + water oxide sulphate

Write out a word equation.

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CuO + H2SO4 CuSO4 + H2O

Write out a proper equation showing the correct formulae.

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CuO + H2SO4 CuSO4 + H2O 64 + 16 (1x1) + 32 + (4x16) 64 + 32 + (4x16) (2x1) + 16

Look up the atomic masses in a table and work out theformula mass of each chemical in the equation.

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80 98 160 18

Look up the atomic masses in a table and work out theformula mass of each chemical in the equation.

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80 98 160 18 178178 178178

This shows that the masses after the reactionare the same as the masses before.

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YIELD AND PERCENTAGE YIELDYIELD AND PERCENTAGE YIELD

YIELD How much of a chemical you getfrom a reaction.

PERCENTAGE How much of a chemical you getYIELD COMPARED WITH how much you

should get. (assuming 100% conversion)

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should get.

Example CuO + H2SO4 CuSO4 + H2O 80 98 160 18


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should get.


If you start with 80g of copper oxide, you canget a maximum of 160g of copper sulphate.


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should get.



Q. What is the maximum yield of copper sulphate if youstart with a) 20g of copper oxide

b) 400g of copper oxide


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should get.



Q. What is the maximum yield of copper sulphate if youstart with a) 20g of copper oxide Ans. 40g

b) 400g of copper oxide


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should get.



Q. What is the maximum yield of copper sulphate if youstart with a) 20g of copper oxide Ans. 40g

b) 400g of copper oxide Ans. 800g


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should get.


Q. If you start with 80g of copper oxide and only get 120g ofcopper sulphate, what is the percentage yield?


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should get.


Q. If you start with 80g of copper oxide and only get 120g ofcopper sulphate, what is the percentage yield?80g should give a maximum of 160g.


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should get.


Q. If you start with 80g of copper oxide and only get 120g ofcopper sulphate, what is the percentage yield?80g should give a maximum of 160g. However only 120gare produced.


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should get.


Q. If you start with 80g of copper oxide and only get 120g ofcopper sulphate, what is the percentage yield?80g should give a maximum of 160g. However only 120gare produced. The fraction produced is 120/160 = ¾ (0.75).


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should get.


Q. If you start with 80g of copper oxide and only get 120g ofcopper sulphate, what is the percentage yield?80g should give a maximum of 160g. However only 120gare produced. The fraction produced is 120/160 = ¾ (0.75).Multiply by 100 to get the percentage.


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should get.


Q. If you start with 80g of copper oxide and only get 120g ofcopper sulphate, what is the percentage yield?80g should give a maximum of 160g. However only 120gare produced. The fraction produced is 120/160 = ¾ (0.75).Multiply by 100 to get the percentage. ANS 75% yieldANS 75% yield


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©©2011 JONATHAN HOPTON & KNOCKHARDY PUBLISHING2011 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

CALCULATIONS INVOLVING CALCULATIONS INVOLVING FORMULAEFORMULAE

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