Empirical and Molecular Formulas - Resources for Teaching Chemistry
Empirical and Molecular Formulas
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Transcript of Empirical and Molecular Formulas
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Empirical and
Molecular Formulas
How to find out what an unknown compound is
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A chemist obtains a new product
What is the formula for the compound?
First step – determine constituent elements and their amounts
This info can be used to determine chemical formula
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Formula of a compound represents relative numbers of atoms present
E.g. “CO2” tells us that in a molecule of this compound there is 1 carbon atom to every 2 oxygen atoms
To determine the formula of a substance we need to count the atoms – we can do this by weighing
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An example
You have a compound that you know contains only Carbon, Hydrogen and Oxygen.
You have a 0.2015 g sample Analysis shows you have 0.0806 g C,
0.01353 g H, and 0.1074 g O We can convert these masses to
moles, and then moles to atoms using dimensional analysis…..
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Determining # of moles of elements in unknown compound
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Determining number of atoms of unknown compound
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To summarize up to this point
We have .00671 moles of Carbon We have .01342 moles of Hydrogen We have .006713 moles of Oxygen
Amount of Carbon = Amount of Oxygen .01342/.006713 = 2, so we have twice
as much Hydrogen as we have of Oxygen and as we have of Carbon
We have a ratio of 1:2:1
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We know the ratio of elements
1 Carbon : 2 Hydrogens : 1 Oxygen
We can write this as CH2O
Is this the molecular formula? Maybe…but the molecule might also have 2 Carbons, 4 Hydrogens and 2 Oxygens, or it might have 16 Carbons, 32 Hydrogens and 16 Oxygens
We have found the EMPIRICAL FORMULA – a formula that represents the ratio of elements in a compound.
This is also called “simplest formula” since it is smallest whole-number ratio of elements in the compound
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Empirical Formula Vs Molecular Formula
Empirical formula gives relative numbers of atoms e.g. CH2O
Molecular formula gives the actual numbers of atoms e.g. C6H12O6
C6H12O6 = (CH2O)6
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Find the Empirical Formula
Empirical Formula of Benzene = CH
Empirical Formula of Dioxin = C6H2Cl2O
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How to calculate empirical formula
We have 0.2636 grams of nickel. We heat it in the presence of oxygen to produce 0.3354 grams of a nickel oxide. What is the formula of the compound we made?
First - what is the mass of oxygen that reacted with the copper?
Mass of copper oxide – Mass of copper = mass of oxygen
0.3354 g copper oxide – 0.2636 g copper = 0.0718 g oxygen
Next – Find number atoms involved…
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Find number of atoms in the compound
•Mole quantities represent number of atoms•We have same number of moles of oxygen as of nickel•Empirical formula will be NiO
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Another example
You have a metal oxide made by reacting 4.151g Al with 3.692 g O. What is the empirical formula?
Atomic mass Al = 26.98 g/mol
Atomic mass O = 16.00 g/mol
Need to know relative numbers of atoms, so need to convert grams to moles and then find the whole number ratio of atoms
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Find whole number ratiosDo this by dividing both numbers by the smallest of the two. This converts the smallest number to 1
This is not a whole number ratio – to get to a whole number, all we have to do is multiply by 2
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Finally we have found our empirical formula
Al2O3
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Summary
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If you have percent composition your can also find the empirical formula
You have a compound that is 27% Carbon and 73% Oxygen by mass. What is the empirical formula?
Assume you have 100 grams. That means you would have 27 g C and 73 g O
Convert grams to moles: 27 g C (1 mol/12 g C) = 2.25 mol C 73 g O (1 mol/16 g O) = 4.6 mol O Divide by smallest # of moles:
4.6 /2.25 = 2.04 (O) 2.25/2.25 = 1 (C)
Empirical formula = CO2
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A poem to help
Percent to massMass to moleDivide by smallMultiply ‘til whole
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Find empirical formula from percent composition
Nylon-6 is a compound that is 63.68 C, 12.38% N, 9.80% H and 14.4% O. Find the empirical formula
C6H11NO
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One more step to find molecular formula
To find the molecular formula of your unknown compound, you need another piece of information – the molar mass of the compound, in addition to the percent composition
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You have the empirical formula – what is the molecular formula?
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Finding molecular formula
The molar mass of the compound will be some multiple of the empirical mass – find the multiple by dividing molar mass by empirical mass:
(238.88 g/mol)/(141.94 g/mol) = 1.7 1.7 ≈ 2 Molar mass of unknown is about twice
empirical mass. We will multiply empirical formula subscripts by 2
Molecular formula = P4O10
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One last problem
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Homework
Read Ch 6, section 3 (pp. 196 – 208) Do empirical formula homework sheets Do molecular formula homework sheets Monday – we will go over homework
and talk about hydrates Test Review Test on Wednesday!!!! We have 12 class meetings to go until
semester final