# chem calculations

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• Created by: reannew
• Created on: 12-03-16 10:45

## the mole

there are many calculations using the mole ;

• number of particles = moles x Avogadro's constant (6.02x10^23)
• number of moles = mass / Mr
• number of moles = concentration (moldm^-3) x volume (dm^3)
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## the ideal gas equation

pV = nRT

p - pressure (pa)

V - volume (m^3)

n - moles

R - gas constant (8.31 jk^-1mol^-1)

T - temperature (k - +273 degrees)

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## masses from equations

• 1 - write out balanced equation
• 2- find the Mr of given compound
• 3- from the given mass, find moles by mass/Mr
• 4- use the ratios in the equation and the calculated moles to find the moles of the required compound
• 5- find the Mr of the require compound
• 6- use the Mr and moles to find the mass by mol / Mr
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## empirical formula by % mass

• 1 - match each element to it's % and find its Mr
• 2- find the moles of each element by % / Mr
• 3 - divide each of the moles by the smallest number of moles you have calculated
• 4- this is the ratio of elements, use this to write the formula
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## empirical formula by experimental data

• 1- find the moles of each element by mass / Mr
• 2- find the ratio of moles by dividing all the mol values by the smallest mol value you have calculated.
• 3- use this to write your emprical formula.
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## concentrations from titrations

• 1- write a balanced equation for the compounds you need
• 2 - work out the moles of the compound given using conc. x vol.
• 3- use the mole ratios from the equation to work out the moles of the required compound
• 4- use conc. = mol / vol to find concentration.
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## % yield

• 1 - find the theoretical yield by:
• a - use mass / Mr to find moles of the given product
• b - use mole ratios from the balanced equation to find moles of required compound
• c - find  the mass by moles x Mr
• 2 - substitute this into this equation:
• % yield = actual yield / theoretical yield
• you will be given the actual yield
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## atom economy

% atom economy = Mr of desired product / sum of Mrs of all reactants x 100

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## enthalpy changes

• 1 - find the bonds broken, use the balanced equation. add their mean bond enthalpy values.
• 2- do the same for bonds made
• 3- minus the value for bonds made from the value of bonds broken to give total change.
• enthalpy change  = total energy absorbed - total energy released.
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