What can the relative atomic masses of the elements in a compound and its formula be used to work out?
Its percentage composition.
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What can we calculate given the masses or percentage composition of elements present?
Empirical formulae.
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How do we calculate the percentage of any of the elements in a compound from the formula of the compound?
Divide the relative atomic mass of the elements by the relative formula mass of the compound and multiply the answer by 100 to covert to a percentage.
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How can it be useful to calculate the percentage of the elements in a compound?
It can be useful when deciding if a compound is suitable for a particular purpose or to identify a compound.
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What is the percentage of carbon in carbon dioxide (CO_2)?
A of C=12, A of O=16, M of CO_2=12+(16x2)=44, So percentage of carbon=(12÷44)x100=27.3%.
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What is the empirical formula?
The simplest ratio of the atoms or ions in a compound.
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What is the empirical formula used for?
It is the formula used for ionic compounds, but for covalent compounds it is not always the same as the molecular formula, e.g. the molecular formula of ethane C_2H_6 but its empirical formula is CH_3.
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How can we calculate the empirical formula of a compound from its percentage composition?
Divide the mass of each element in 100g of the compound by its A to give the ratio of atoms. Then convert this to the simplest whole number ratio.
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What is the empirical formula of the hydrocarbon that contains 80% carbon?
Mass in 100g of compound= carbon 80 : hydrogen 20. Ratio of atoms or moles of atoms (Mass÷A) carbon 80÷12=6.67 hydrogen 20÷1=20. Simplest ration of atoms (divide by smallest) carbon 6.67÷6,67=1 hydrogen 20÷6.67=3. Empirical formula =CH_3.
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What is the percentage of carbon in methane CH_4? (A of C=12, A of H=1)
(12÷16)x100=75%.
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What is the empirical formula of the compound that contains 70% iron and 30% oxygen? (A of Fe=56, A of O=16)
70÷56=1.25 : 30÷16=1.875, 1:1.5, 2:3, empirical formula Fe_2O_3.
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**WHEN CALCULATING AN EMPIRICAL FORMULA IT IS HELPFUL TO SET OUT YOUR ANSWER IN A TABLE, IN THE EXAM YOU SHOULD ALWAYS SHOW YOUR WORKING IN CALCULATIONS**
**PRACTICE CALCULATING THE PERCENTAGE OF AN ELEMENT IN A COMPOUND FRO ITS FORMULA**
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**YOU SHOULD BE ABLE TO CALCULATE THE EMPIRICAL FORMULA OF A COMPOUND WITH TWO OR THREE ELEMENTS FROM INFORMATION ABOUT ITS PERCENTAGE COMPOSITION IF YOU ARE SITTING THE HIGHER TIER**
**A SMALL DIFFERENCE IN THE AMOUNT OF METAL IN AN ORE MIGHT NOT SEEM VERY MUCH BUT WHEN MILLIONS OF TONNES OF ORE ARE EXTRACTED AND PROCESSED EACH YEAR IT AMOUNTS TO A LOT**
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Other cards in this set
Card 2
Front
What can we calculate given the masses or percentage composition of elements present?
Back
Empirical formulae.
Card 3
Front
How do we calculate the percentage of any of the elements in a compound from the formula of the compound?
Back
Card 4
Front
How can it be useful to calculate the percentage of the elements in a compound?
Back
Card 5
Front
What is the percentage of carbon in carbon dioxide (CO_2)?
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