Chemical Calculations

7/17/2019 Chemical Calculations

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Balancing equations

An important principle in chemical reactions is that matter

cannot be created or destroyed. t is important that symbol

e!"ations are balanced.

A balanced e!"ation has the same n"mber of each type of

atom on each side of the e!"ation.

Na + Cl2 NaCl

1 sodium 2 chlorine 1 sodium 1 chlorine

Balanced#

$his shows that two moles of sodi"m react with one mole

of chlorine to make two moles of sodi"m chloride.

2Na + Cl2

2NaCl

2 sodium 2 chlorine 2 sodium 2 chlorine

%nbalanced#


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Balancing unfamiliar equations


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Balancing ionic equations

'!"ations containin( ions sho"ld ha)e the same o)erall

char(e on each side in order to be balanced.

$his can be achie)ed by balancin( the e!"ation in the

normal way#

Ca2+

+ Cl-

CaCl22 calcium 1 chloride 2 calcium 2 chloride

*1 char(e no char(e

2 calcium 2 chloride 2 calcium 2 chloride

Ca2+ + 2Cl- CaCl2

no char(e no char(e

Balanced#

%nbalanced#


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Balancing ionic equations problems


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State symbols

State symbolsare letters that are added to a form"la to

indicate what state each reactant and prod"ct is in.

$he fo"r state symbols are#

$hese are added after the form"la in brackets and s"bscript.-or eample#

2H2g! + "2g! 2H2"g!

s

l

g

aq

solid

liquidgas

aqueous


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#dding state symbols


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$eacting masses



Calculating reacting masses

$o calc"late the mass of a prod"ct (i)en the mass of a

reactant "se the followin( steps#

1% Calculate no% moles of reactant&

no. moles mass Mr

2% 'etermine mole ratio of reactant to product&ens"re the e!"ation is balanced

(% Calculate no% moles of product&

"se the mole ratio

)% Calculate mass of product&

mass moles Mr



$eacting masses e*ample

4hat mass of sodi"m chloride is prod"ced if 2.30 ( of

sodi"m is b"rnt in ecess chlorine

(%6alc"late

no. moles of 7a6l#

%1 moles Na , %1 moles NaCl

)%6alc"late

mass of 7a6l#

mass , moles Mr, %1 ./%.

, .%/.g

1%6alc"late

no. moles of 7a#

no% moles , mass 0 Mr, 2%( 0 2(%

, %1

2%8etermine mole ratio of 7a to 7a6l#

2Na + Cl2 2NaClratio , 2&2

, 1&1



$eacting masses calculations


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ore reacting masses calculations


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hat is concentration3

$he concentrationof a sol"tion is a meas"re of how m"ch

soluteis dissol)ed per "nit of sol4ent.

amo"nt of sol"te is meas"red in moles

)ol"me of sol)ent is meas"red in dm3

concentration is meas"red in moldm3.

concentration , amount of solute 0 4olume of sol4ent

:ol"mes are often epressed in cm3 so a more "sef"l

e!"ation incl"des a con)ersion from cm3to dm3.

concentration , no% moles 1! 0 4olume

moldm3 cm3


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Concentration5 moles and 4olume


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Concentration calculations



Standard solutions

A standard solutionis a sol"tion of known concentration.

;tandard sol"tions are made by

dissol)in( an acc"rately wei(hed mass

of solid in a known )ol"me of sol)ent

"sin( a 4olumetric flas6.

$he )ol"metric flask has a thin neck

which is marked with a line so it can be

filled acc"rately to the correct capacity.

$he standard sol"tion can then be "sed to find the

concentration of a second sol"tion with which it reacts.

$his is known as 4olumetric analysisor titration.



7reparing standard solutions



A titrationis a proced"re "sed to identify the concentration

of a sol"tion by reactin( it with a sol"tion of known

concentration and meas"rin( the )ol"me re!"ired for a

complete reaction.



Carrying out a titration



8itration calculations e*amples

4hat is the concentration of an 7aed by 23.&cm30.99/moldm3=6l sol"tion

(%6alc"late no.

moles of 7a


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8itration calculations



ore titration calculations


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hat are the different types of yield3

$he theoretical yieldis the maim"m mass of

prod"ct epected from the reaction calc"lated

"sin( reactin( masses.

$o calc"late the percenta(e yield the theoretical yield

and the actual yieldm"st be calc"lated.

$he actual yieldis the mass of prod"ct that is

act"ally obtained from the real chemical reaction.

$he percentage yieldof a chemical reaction shows

how m"ch prod"ct was act"ally made compared withthe amo"nt of prod"ct that was epected.

C l l i i ld


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Calculating yield

$he percenta(e yield of a reaction can be calc"lated "sin(

the followin( e!"ation#

percentage yield , actual yield 1! 0 theoretical yield

;*ample& hat is the percentage yield of a reaction


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hat is atom economy3

#tom economyis another meas"re of the efficiency of a

chemical reaction. t is the mass of reactants that end "p as

the desired prod"ct this is calc"lated as a percenta(e.

$his concept is "sef"l to chemical ind"stry beca"se it

takes into acco"nt the atoms that end "p in "nwanted

waste prod"cts as well as the yield of the reaction.

$his means a process that prod"ces se)eral worthless

byprod"cts co"ld ha)e a hi(h yield b"t a low atom economy.

eactions with a hi(h atom economy tend to be more

en)ironmentally friendly as they tend to prod"ce less waste

"se fewer raw materials and "se less ener(y.

C l l ti t


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Calculating atom economy

;*ample& hat is the atom economy of a reaction


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?ield and atom economy calculations


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@l


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@lossary

h tA th 6 d3


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hatAs the 6ey


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ultiple-choice qui

Chemical Calculations

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