Chemical calculations grade 11 (Stoichiometry)

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Chemical Calculations (Stoichiometry)

K Warne

http://www.warnescience.net/



Atomic WeightsYou must be able to…· Describe the mole as the SI unit for amount of substance

· Relate amount of substance to relative atomic mass

· Describe relationship between the mole and Avogadro’s

number

· Conceptualise the magnitude of Avogadro’s number

· Describe the relationship between molar mass and

relative molecular mass

· Calculate the molar mass of a substance given its

formula




REACTIONSHydrogen and oxygen react to produce water.

(Synthesis reaction)

H2 + O2 H2O

Ar: H = 1, O = 16

Mr: H2 =2, O2 = 32

H2O = 16+2=18

Balanced Eqn:

2H2 + O2 2H2O

LHS: RHS:

2(2) + 32 = 36 2(2+16) = 36

LHS = RHS

MASS is CONSERVED!!

This is true for ALL REACTIONS.

This is the reaction used to

power the space shuttle.




ASKEDGIVEN

Mole Calculations

MOLES MOLES

MASS MASS

VOLUME VOLUME

MOLAR

RATIO

Number

Of

particles

Number

Of

particles




The MoleThe mole is defined as, “the amount of ………….. with the same number of

……………………… particles as ….. grams of carbon 12”. (n used as symbol for moles)

602 300 000 000 000 000 000 000Six hundred and two thousand, three hundred, billion billion !

6.023x1023 particles

12.00 g

CSymbol (….)

Number of particles = no of moles x no. particles in a mole

Particles = ……………..




Moles & Particles

...........................................................................

...........................................................................

...........................................................................

...........................................

...........................................................................

...........................................................................

...........................................................................

...........................................

...........................................................................

...........................................................................

...........................................................................

...........................................particles

1 mol 1 mol 1 molmoles

(n)

X

L

6.023 x 1023

18.069 x 10236.023 x 1023

6.023 x 1023 6.023 x 1023

?3




The Mole and MassThe mole is defined in such a way that the MASS NUMBER (A) of an element

is equal to the relative atomic mass mass of one mole of the substance. (in

grams) - THE MOLAR MASS

• Eg Na = 23g/mol, water(H2O)=18g/mol

Z

A

XAtomic Number

(smaller)

Mass Number

(bigger)

protons + neutrons

Periodic Table Symbol

Relative atomic

mass

or

mass(g) of one

mole




Isotopes

• Isotopes - Atoms of the same element which have different

numbers of neutrons. Eg: 613C & 6

12C

Relative atomic mass is (actually) the average mass (of all the isotopes in a

random sample) of the atoms of an element relative to 1/12 of the mass of

a carbon-twelve atom.

613C

• 6 protons

• 6 electrons

• 13-6 = 7 neutrons

612C

• 6 protons

• 6 electrons

• 12-6 = 6 neutrons




The Mole - equationsSodium reacts with water to form hydrogen and sodium hydroxide

according to the equation.

Na + H2O H2 + NaOH

If 46g of sodium are reacted with excess water what mass of hydrogen would be formed?

1. Balance the reaction

2Na + 2H2O H2 + 2NaOH

2 Work out moles of reactant (given).

n(Na)=m/Mr=46/23=2mol

3 Go through the equation to find out the number of moles reacting and being formed - the molar ratio:

Na : H2 2:1 => 1 mole H2 formed

4 Work out quantity asked for.

m(H2) = nxMr = 1 x 2 = 2 g




Mole examples - B & J p119 21 & p120 22

1. Na + Cl2 NaCl

Calculate the mass of salt formed if 2.3g of sodium is reacted with XS chlorine.

2. Zn + HCl ZnCl2 + H2

What mass of HCl is needed to produce 100g of hydrogen?

3. KClO3 KCl + O2

What mass of oxygen is produced from 1kg of potassium chlorate?

4. Fe2O3 + H2 Fe + H2O

What mass of iron is produced if 3g of rust (Fe2O3) is reacted with XS(100g )of hydrogen?




Mole examples - B & J p119 21 & p120 22

1. 2Na + Cl2 2NaCl

Calculate the mass of salt formed if 2.3g of sodium is reacted with XS chlorine.

1. n(Na) = m/Ar = 2.3/23 = 0.1mol

2. Molar Ratio

Na:NaCl 2:2 ie 1:1 => n(NaCl) = 0.1mol

3. m(NaCl) = nxMr = 0.1x(23+35.5) = 5.85g




Limiting reagentIf 20 g of Na reacts with 20 g of water – what

mass of hydrogen will be formed?




Percentage CompositionAnalysis of a compound by mass makes it

possible to work out the % mass of each element.

eg Table salt: NaCl mass analysis:One mole of NaCl would have a mass of

23 + 35.5 = 58.5g

• The % composition can be found using the formula:

Mass element X x100

Total Mass Compound

• %Na = […../ (…..) ]x100 = …………..% (by mass)

• %Cl = (…../ (…….) )x100 = …………%

% Mass Element X =




Empirical and Molecular Formula.A compound consists of carbon, hydrogen and oxygen only. The

% by mass are Carbon 40.0% and 6.7% hydrogen. Calculate the

empirical and molecular formula of the compound if Mr = 60g·mol-1

%(O) = 100 – (40+6.7) = 53.3

C H O

In 100g: …….g ……..g ….…g

n=m/Mr:…/…

6.7/….53.3/……

…… …… ……..

……. …… …….

Simplest: … …… ….

Empirical Formulae:

……. (12+2+16 = …..)

Molecular Formula: 2(CH2O)

……… (Mr = …. X 30)




Empirical and Molecular Formula.A compound consists of carbon, hydrogen and oxygen only. The

% by mass are Carbon 40.0% and 6.7% hydrogen. Calculate the

empirical and molecular formula of the compound if Mr = 60g·mol-1

%(O) = 100 – (40+6.7) = 53.3

C H O

In 100g: 40.0g 6.7g 53.3g

n=m/Mr:40/12

6.7/153.3/16

3.33 6.7 3.33

3.33 3.33 3.33

Simplest: 1 2.01 1Empirical Formulae:

CH2O (12+2+16 = 30)

Molecular Formula: 2(CH2O)

C2H4O2 (Mr = 2x30)




Concentration - MolarityThe concentration of a solution is defined as the ………………. of

……………………… per ………………. (dm3) of ………………….

solute

solute

Final volume of

……………..

500cm3

=+

Concentration =Amount of ……… (……….)

Volume of ………………

30g of

NaCl

C =n

v




ASKEDGIVEN

Mole Calculations

MOLES MOLES

MASS MASS

VOLUMEVOLUME

CONCENTRATIONCONCENTRATION

MOLAR

RATIO

Number

Of

particles

Number

Of

particles




Volume Conversions

1 dm = ….. cm

1 dm3 = ………… cm3

1 m3 = …………….. dm3 = ………………….cm3 (10….)

1cm3

1 dm3 (1 litre)

10 cm3

10 cm3

10 cm3




Decimal Conversions

King Henry Died a miserable death called measles

Kilo Hecta Decca m(unit) deci centi milli

1000 100 10 1 1/101/100

1/1000

1m

?km

0. 0 0 1km

1 m = 0.001 km

1 km = 1000 m




Conversions

1 cm = 0.1 dm

1 cm2 = (0.1)2 dm2

1 cm2 = 0.01 dm2

1 cm3 = (0.1)3 dm3

1 cm3 = 0.001 dm3

1 dm3= 1000 cm3

25 cm3 = 0.025 dm3




Molar VolumesOne mole of an ideal (ANY) gas occupies a volume of ………….3 at

………………………… temperature and pressure. (STP)

STP: T= ….ºC, ……K P =1 atmosphere (……...kPa)

Fe2O3 + 3H2 2Fe + 3H2OWhat volume of hydrogen reacts with 50g of Fe2O3

Fe2O3 : H2

… : …..

n(H2) =…..n(Fe2O3) = ……………………

v(H2) = …………………………… dm3

n(Fe2O3) = m/Mr = ………………….= ………………mol

moles = volume/molar volume ==> n = …./…..




Concentration CalculationsHydrochloric acid reacts with calcium carbonate according to the following

equation:

HCl + CaCO3 CaCl2 + H2O + CO2

What mass of calcium carbonate would be needed to react completely with 25 cm3 of a 0.1 mol.dm-3 hydrochloric acid solution?




Volume - Volume Calculations1. Balance the equation

2. Calculate the moles of the substance given.

3. Work through the molar ratio to find out the moles of the substance asked.

4. Calculate the quantity asked for. (Volume V = n x Mv)

Mv = 22.4dm3 At STP

EG: H2 + N2 --> NH3If 3.00 dm3 of nitrogen are reacted to produce ammonia, what

volume of hydrogen will be required? (At STP)

H2 + N2 --> NH3




Mass Volume Calculations1. KClO3 KCl + O2

What volume of oxygen is produced by the decomposition of 1kg of potassium chlorate?

1. Balance the equation - 2KClO3 2KCl + 3O2 (1)

2. Calculate the moles of the substance given.

n(KClO3) = m/Mr = 1000/(39+35.5+3(16)) = 8.16mol (1)

3. Work through the molar ratio to find out the moles of the substance asked.

KClO3 : O2 2 : 3

n(O2) = 3/2n(KClO3) = 3/2(8.16) = 12.24 mol (1)

4. Calculate the quantity asked for. (Volume V = n x Mv)

Mv = 22.4dm3 At STP

v(O2) = n(O2)Mv = 12.24(22.4) = 275 dm3 (2)




Standard Solution

A standard solution is one for

which the concentration is precisely known.

Since

c = n(solute)/v(solvent)

= m/Mr V

• The number of moles of solute(Mass)

• The volume of solution.

These values must be accurately determined.

2.45g

Mass is determined accurately

using an electronic balance.

• Possible accuracies of 0.1 -

0.0001g

KMnO4

Volume is measured

using a volumetric

flask.

• 250 cm3

• 100 cm3, 200 cm3,




Volumetric FlaskMaking a standard solution.

1. Rinse a clean & dry 100 cm3 beaker with a little distilled water.

2. Transfer the correctly weighed amount of salt to the beaker. Ensure NO SALT IS LOST.

3. Add 50 - 80 cm3 water the salt and stir gently with a glass rod until all salt is dissolved. DO NOT REMOVE THE ROD FROM THE SOLUTION NOR ALLOW ANY DROPS OF SOLUTION TO ESCAPE.

4. Add ALL the solution to volumetric flask via funnel. Ensure glass rod and beaker are thoroughly rinsed. (Include rinsings.)

5. Add enough solvent to bring the level up to the mark.




Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain a lot more more slides and other resources, are freely

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Have a look and enjoy!

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Chemical calculations grade 11 (Stoichiometry)

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Transcript of Chemical calculations grade 11 (Stoichiometry)