Chemistry chemical calculations

Chemical calculations

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  • Created by: Laura
  • Created on: 09-05-12 18:12

The mole

Relative atomic mass (Ar) is the mass number of an element. It  shows how heavy different atoms are compared to the mass of carbon-12

Relative formula mass (Mr) is the Ar of all the elements in  the compound combined. E.g. the Mr of MgCl2 is 24+(35.5x2) = 95

The mole

The mole is a specific number (6.023 x 1023)

One mole of an element weighs exactly the Ar of the same element in grams (e.g. 1 mole of carbon-12 weighs 12g.....or..... 1 mole of CO2 weighs 44g because the Mr is 44. 

Number of moles = mass (g) of element/compound ÷ Mr (of element/compound)


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Percentage composition

Percentage mass = Ar x Number of atoms (of that element)

                               ----------------------------------------------------         x 100

                                      Mr of whole compound

E.g. The percentage of oxygen in NaOH 

Ar ... Na=23 , O=16 , H=1

     16 (x1)

     ------------      x 100       = 40%

    (23+16+1)

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Empirical formula

Empirical formula is the simplest version of a compound formula. 

1) List all elements in the compound  

 2) Underneath write their experimantal masses      

3) Divide the experimantal mass by the Ar for that element

4) Find the ratio (divide by the smallest number) 

E.g. to find the empirical formula of the iron oxide produced when 44.8g of iron react with 19.2g of oxygen. (Ar for iron = 56 , Ar for oxygen = 16.)

1) Fe ....... O            2) 44.8 ....19.2         3) 44.8 ÷ 56 = 0.8         19.2 ÷ 16 = 1.2

4) Ratio = 0.8 : 1.2 (x by 10 to get whole numbers) = 8 : 12  = 2:3

EMPIRICAL FORMULA = Fe2O3

To find the molecular formula from this, divide the Mr for the molecular formula (from q) by the Mr for the empirical formula. Multiply the numbers in the empirical formula by the answer.

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Reacting Masses

Mass is never lost or gained in a chemical reaction, it is conserved. The total mass of reactants is always the same as the total mass of the products.

1) Write out a balanced equation

2) Work out the Mr (for the bits you want)

3) Divide to get one gram, then multiply to get all

 E.g. How much MgO is produced when 60g Mg is burned in air?

1) 2Mg +O 2 ---> 2MgO

2) Mr for 2Mg = (2x24) = 48     Mr for 2MgO = (40x2) = 80

48g of Mg react to give 80g of MgO

3) 1g Mg = 48 ÷ 48 = 1              MgO = 80 ÷ 48 = 1.67           1g Mg ---> 1.67g MgO

1.67 x 60g = 100g     

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Percentage Yield

100% percentage yield is not possible because:

  • if the reaction is reversible (e.g the haber process),the products can react again to form the initial reactants, and yield may be lowered by changing equilibrium position,
  • When you filter a liquid to remove solid particles, a bit of liquid/solid is always lost. Liquid may be lost as it remains with the solid (the solid is wet). Solid may be left behined as it is hard to scrape off the filter paper
  • Transferring liquids from one container to another always looses a bit of liquid (the inside surface is always wet)
  • Unexpected reactions may occur, so the yield of the intended product decreases. Impurities in the mixture may change the reeaction.
  • If more than one product is produced, it is difficult to separate the product that we want from the reaction mixture.

Percentage Yield = actual mass of product obtained (grams)

                              -----------------------------------------------------   x 100

                               predicted yield (theoretical mass) (grams)

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Atom Economy

Atom economy is a measure of what percentage of the reactants reacted to become useful products. 

Atom economy =  Total Mr of useful products

                             ---------------------------------        x 100

                               Total Mr of reactants

E.g. Hydrogen gas is made by reacting methane with steam. Calculate atom economy.   CH4 + H2O --->CO + 3H2

(1x2x3)  ÷  (12 + (1x4) +(1x2) + 16)   x 100       = 17.6%   a low atom economy. 

A low atom economy is wasteful. These reactions use up resources very quickly, and the waste materials must be disposed of (pollution.) These reactions are unsustainable. 

They aren't very profitable because raw materials are expensive to buy, and it can be expensive to responsibly remove waste products. 

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