5.Finding mass constituents using formulae.
This is a revised translation of Chapter 5 from the Sinhalese book "Step by step Chemical Calculations" earlier published by the author.
https://www.youtube.com/watch?v=IJpZsvYygF8
In Industry chemical compounds are used to get various substances necessary for us. For example we get lime(Calcium oxide) from corals. Dead corals from the sea or earth are almost a pure form of Calcium carbonate. Lime is used to make cement. Chemical engineers need the exact mass of corals that have to be heated to get the required amount of lime.
For this purpose they make use of the chemical formulae and the Relative atomic masses.
Example 1.
Question:- Find the maximum mass of Chlorine that can yield from 12 kg of Sodium chloride.
Answer:- Relative molar mass of NaCl = 23 + 35.5 = 58.5
According to this 58.5 kg of NaCl should give 35.5 kg of chlorine
1 kg of NaCl should give = 35.5 / 58.5 kg of chlorine.
12 kg of NaCl should give = 35.5 x12 / 58.5 = 7.28 Kg.
Please note that, it is usually not possible to obtain all the chlorine available as there can be some wastage.
Using the formulae:
Mass = 35.5 x 12 /58.5 = 7.28kg.
1. Find the amounts of the elements formed by the electrolysis of water.
| Question |
Answer |
Mass of Hydrogen from 18 g*. of H2O.
*molar mass of water.
Mass of Hydrogen from 1 g. of water
Mass of Hydrogen from 100g. of water Mass of Oxygen from 100g of water. |
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= ---------
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2. Find the mass of Oxygen in 27g of HgO.
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Mass of Oxygen in 201+ 16= 217g of HgO
Mass of Oxygen in 1g of HgO.
Mass of Oxygen in 27g. of HgO.
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3.Find the mass of silver that may be obtained from 100g of silver nitrate.
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Molar mass of AgNO3
Mass of silver from 138g of AgNO3
Mass of silver from 1g of AgNO3
Mass of silver from 100g of AgNO3
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4. Find the mass of Carbon dioxide that can be obtained by heating 100g of baking powder.NaHCO3
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Molar mass of Lime stone NaHCO3 .
Mass of CO2 in 72g of the chemical
In 100g
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5. A sample of lime stone was found to contain 90% of CaCO3. Find the mass of CaO in 1 tonne of lime stone.
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Pure CaCO3.
In 100 tonnes of sample
200 tonnes ……………………………..
CaO in100 tonnes of pure CaCO3 has
Therefore 180 tonnes of CaCO3.
Therefore 200 tonnes of sample will yield
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6 Agricultural fertilisers usually display a NPK value. These are related to the percentages of Nitrogen, Phosphorus and Potassium present. P actually gives the phosphorus pentoxide( P2O5) value. If a fertiliser has 30 .15 10 What will be mass of Phosphorus in a 10kg fertiliser bag.
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Given Percentage of P2O5 in the 10 kg bag
Mass of P2O5 in bag.
Molar mass of P2O5
Mass of pure P in 142 kg of P2O5 will have
1.5 kg of P2O5 will give
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15%
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