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At the beginning of the lesson, student will be "hooked" by a short CSI clip, which shows "mysterious white powder". Students will be told that researchers used various analytical methods to find out the powder's chemical composition, but they are sure about the powder's identity. Students will then learn how to determine and write chemical formula of various chemical substances. The lesson will then be tied back to its beginning and students will be asked to determine the correct chemical formula for the "white powder".
After learning about empirical and molecular formulas, SWBAT accurately explain the difference between empirical formula and molecular formula.
Given the percentiles of individual components of a molecule (ionic compound), SWBAT come up with the compound’s empirical formula with 100% accuracy.
Given an empirical formula of a compound and its molar mass, SWBAT come up with its molecular formula with 100% accuracy.
After the lesson, SWBAT determine the identity of “suspicious powder” (its molecular formula).
After the lesson, SWBAT explain the aspect of science that played a significant role in determining the identity of “suspicious power” with 100% accuracy.
Standards of Learning:
The student will investigate and understand how conservation of energy and matter is expressed in chemical formulas and balanced equations. Key concepts include
c) writing chemical formulas (molecular, structural, and empirical; and Lewis diagrams);
“Suspicious power” was discovered, broken down to individual atoms, and analyzed. What is the identity of this compound that consists of 40% Ca, 12 % C, 48% O?
The identity of this compound can be discovered by determining the combination of these individual elements. This combination can be written as a chemical formula. In other words, a chemical formula is used to represent a specific compound.
There are two types of chemical formula:
1. Empirical Formula – Represents the lowest whole number ratio of elements in a
molecular compound. Equivalent to the formula unit for ionic compounds.
2. Molecular Formula - Represents the actual number of atoms present in a molecular compound.
An empirical formula may represent several different compounds, while a molecular formula will only represent one (except in organic chemistry).
Empirical Formula Molecular Formula
Q. Can we see the difference between formaldehyde and glucose by just looking at their empirical formula?
Q. How about by looking at their molecular formulas?
Q.Is it safe to say that empirical formulas are just representation of ratios between individual components?
What is the empirical formula of a compound with a molecular formula of C2H4?
Which of the following represents an empirical formula/molecular formula pair? Explain.
There are steps to determine an empirical formula:
1. If given percentages, change percentage unit to unit of grams.
2. Convert to moles.
3. Divide each by the smallest number of moles to get the ratio.
4. Multiply to get whole numbers if necessary.
5. Answers in #4 become subscripts.
Also, there are steps to determine a molecular formula:
Steps to calculate a molecular formula:
1. Calculate the empirical formula as above.
2. Calculate the molar mass of the empirical formula.
3. Divide the mass of the molecular formula (given) by the mass of the empirical
4.Multiply the subscripts in the empirical formula by the answer in step 3.
We have a compound that is composed of 36.5% sodium, 25.4% sulfur, and 38.1% oxygen.
Determination of an empirical formula
1. For the first step, we can change % unit to g unit, assuming that we have 100g of the compound.
2. Now we can determine the number of moles for each compound.
(36.5 g Na) (1 mole Na/23g)
= 1.6 mole Na
(25.4 g S) (1 mole S /32g)
= 0.79 mole S
(38.1 g O)(1 mole O/ 16g)
=2.4 mole O
3.Divide each by the smallest number of moles to get the ratio.
1.6 mole Na/0.79 = 2
0.79 mole S/0.79 = 1
0.79 mole O/0.79 = 3
The empirical formula is Na2SO3
Determination of an molecular formula
1, 2 The formula mass of the empirical formula in the sample problem above is 126 g/mol.
3. If the molecular mass is known to be 378 g/mol so, (378 g/mol) /(126g/mol) =3
4. Multiply each subscript in the empirical formula by this whole number.
We now have the molecular formula - Na6S3O9
Q. So we formula should be determined first?
A. Empirical formula.
Q. But does an empirical formula give the exact identity of the molecules? If now, what do we need to know more?
A. Molecular formula.
Q. What do we do first if we were given the mass of each component instead of percent?
A. Do the same thing, but use the given mass to calculate the moles. We wouldn’t have to assume that there is 100g of the compound because we already know the mass.
Individual practice: The 5 practices on the slides.
- Answer, go-overs.
So today, we learned about empirical formula and molecular formula. Now using this information, let’s determine the identity of the suspicious powder. We can first determine empirical formula using our white stuff from the crime scene.
The compound has 40% Ca, 12 % C, 48% O.
(40 g Ca ) (1 mole Ca/ 40.078g Ca)
=1 mole Ca
(12 g C) (1 mole C /12. 011g)
= 1 mole C
(48 g O)(1 mole O/ 15.99g)
=3 mole O
1 mole Ca/1 = 1
1 mole C/1 = 1
3 mole O/1 = 3
Empirical formula: CaCO3
The powder was from a science teacher’s chalk!
Nature of science: Process skills are practical tools.
Method(s) of Assessment:
Homework worksheet (practice on writing empirical formula and molecular formula)
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