Mass of one grape is 2g.
We have bought 500g of strawberries and the
same mass of grapes.
Can you find the number fruits in each category?
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Na
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C
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O
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H
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Cu
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Ca
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Pb
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Hg
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23
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12
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16
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1
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64
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40
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207
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200
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Fig.1 Grapes and strawberries.
Calculating the number of fruits.
Mass of straw berries …………….= 500g
Mass of one berry………………….= 10g
Number of berries = 500g ÷ 10 g = 50 fruits.
Mass of one grape …………………= 2g
Number of grapes ………………..= 500g ÷ 2 = 250 fruits
The number ratio of fruits strawberries : grapes = 50 : 250 = 1: 5
If you are to packet lot according to availability It should be in this ratio.
Each packet will have = strawberry 1 grapes5
This is similar to a chemical formula.
A chemical consisting of three elements had the following percentages by mass.
Na = 43.40 % C = 11.30 % O = 45.3 %
Find the formula of the compound.
Mass ratios of the elements………………= 43.4 : 11.3 : 45.3
Number ratio……………………………......=43.4/ 23 : 11.3/ 12 : 45.3/ 16
23,12 and 16 are the atomic masses of Na, C and O.
Converting fractions to decimals…………= 1.89 : 0.942: 2.83
Divide these numbers by the smallest value, which is 0.942.
The we get ……………………………….= 2: 1 : 3
Formula of the chemical is = Na2 C1 O3
(We don’t have to write the 1 for C. )
This is the formula of Sodium carbonate.
It will be interesting to find the formulae of these;
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Question
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Mass ratio
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Number ratio
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Formula
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1.0
Mass of Mercury oxide 54 g
Mercury formed after heating = 50 g
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Hg : O
50 : 4
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…./200 … : …../ 16
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2.0
340 g of a compound containing only Hydrogen and Oxygen was decomposed.
Mass of Oxygen obtained = 320 g
Mass of Hydrogen = …………g
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H : O
….: ……
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…..÷ …..: …..÷ ……..
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3.0
When 1.6g of Copper was heated in air mass increased to 1.8g. Assumig that Oxygen combined with copper, find the formula of copper oxide.
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Cu : O
…………
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…./64 : ……/ 16
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4.0
A compound has 54.05% of Calcium, and 43.24% Oxygen. Assuming the rest is Hydrogen Find the formula of the compound.
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Ca: O: H
………….
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…../.: …./ …. : …/ ….
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5.0
By the reduction of 2.39g of Lead oxide, 2.07g of Lead was recovered.
Find the probable formula of lead oxide.
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Pb : O
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Answers:1. HgO, 2. H1O1, (There is no compound like this. This compound really is H2O2,) 3. Cu2O, 4. Ca O2H2 (This really is Ca(OH)2 , 5. PbO
