Unit 3 lesson 4

Unit 3 lesson 4

Percent Composition By Mass

• To find the percentage composition of an element in a compound, first find the molar mass of the compound.

• Then divide the element mass by the compound mass.

• (Think of it this way: What percentage of the total mass of the compound comes from that specific element?)

At the top is the formula for percent composition

Example Problem

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Step 1

• Calculate the molar mass of the compound

• ID the ratio of elements in the compound

• Multiply their independent molar masses by their ratios

• Add the values together

Step 2

• Divide the independent molar masses of each element over the total molar mass of the compound

• Than multiply those numbers by 100%

Simplest (Empirical) Formula

• shows the smallest whole # ratio of atoms in a compound.

• For example, the empirical formula of glucose (C6H12O6) would be CH2O.

Practice Problem

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Multiple Choice

What would be the Empirical Formula of C₆H₁₈O₃

• CHO

• C₆H₁₈O₆

• C₂H₆O

• C₃H₆O

There are three steps for calculating the empirical formula in most problems:

• 1st – change % to grams 

• 2nd – convert grams to moles

• 3rd – divide all your answers by the smallest # of moles

Keep this in mind

Usually this will give you a set of numbers that are very close to whole numbers (they could be slightly off, like 4.03 instead of exactly 4, due to rounding or experimental error), and you would simply round them off to get your final answer.

Sometimes, however, you have to look at your answers and use a bit of common sense. If one of your answers ends in something very close to “.5”, you don’t round – you multiply all your numbers by 2. Or if you get a number that ends in something like “.33”, you’d multiply everything by three to clear it up.

Example Problem

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Step 1

• Convert percents to grams

Step 2

• Convert grams to moles

Multiple Choice

Which element has the smallest number of moles?

• Boron - 7.22 mol

• Hydrogen - 21.8 mol

Step 3

• Divide all your answers by the smallest # of moles 

• those numbers are our subscripts for B and H

Molecular Formula

•  indicates the actual # of each type of atom in the compound, and not just their ratios.

• most sugars have the simplest formula CH2O; but glucose is C6H12O6 (same ratio, but not exact same formula: CH2O is the simplest formula; C6H12O6 is the molecular formula)

Multiple Choice

Is C₁₀H₂₂ the molecular formula of C₅H₁₁?

• Yes

• No

Example Problem

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Calculate Molecular Formula

• Step 1: Calculate the molar mass of the Empirical Formula

• Step 2: Divide the molar mass of the molecular formula over the molar mass of the empirical formula

• Your answer is the amount you should multiply your empirical formula's subscripts by

Step 1

• Calculate the molar mass of the Empirical Formula

Step 2

• Dive the molar masses

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