Percent Composition and Chemical Formulas

Wednesday 11/16: Unit 5 calculating empirical formulas

Announcements/Reminders

Agenda:

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• Phones up by the bell!

• 3 in person school days until Thanksgiving break!!!

• Friday 11/18- Unit 5 test

• Work on study guide and unit 5 test review​

• If you plan to be absent on Friday please let me know and make arrangements with me to take the test on Thursday before school (PM students) or after school (AM students).

• Mon/Tues ​11/21-11/22 are elearning days, check GC

Bellwork:

On GC

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• bellwork

• calculating empirical formulas from per​cent composition

• lego activity

​Sometimes, the empirical formula is also the molecular formula ( ex. H₂O)

Multiple Choice

A formula with the lowest whole # ratio of elements in a compound is called

• Molecular Formula

• Chemical Formula

• Empirical Formula 

• Distance Formula

Multiple Choice

What is the empirical formula for C₄H₆?

• CH

• CH₃

• C₂H₃

• C₄H₆

Multiple Choice

What is the empirical formula for N₂S₃?

• NS

• N₃S₂

• N₂S₃

• N₁S_1.5

Multiple Choice

Which pair has the same empirical formula?

• NaCrO₄ and Na₂Cr₂O₇

• C₂H₄O₂ and C₆H₁₂O₆

• C₃H₆O₃ and C₂H₆O₂

• CH₄ and C₂H₆

​How to calculate empirical formula from percent composition

In order to calculate the empirical formula of a compound we must first determine the relative masses of elements present- 

• The relative masses will give us what?

  •  the ratios of the elements present in the compound. 

• The ratios will give us what?

  •  the subscripts

• One way to find the relative masses in the compound is to measure the masses that react to form the compound

• Write this down: Percent to grams, grams to moles, divide by smallest, multiply til whole

​Percent to grams, grams to moles, divide by smallest, multiply til whole

Multiple Choice

What is the first step in calculating empirical formula?

• convert grams to moles

• change percent to grams

• divide by smallest number of moles

• multiply 'til whole

Multiple Choice

How many moles are in 83.65 g of carbon?

• 1,003.8

• 6.965

• 501.9

• 83.65

Multiple Choice

How many moles are in 16.35 g of hydrogen?

• 8.175

• 16.22

• 16.5

• 32.7

Multiple Choice

What is the next step in calculating empirical formula?

• convert grams to moles

• change percent to grams

• divide by smallest number of moles

• multiply 'til whole

​Percent to grams, grams to moles, divide by smallest, multiply til whole

​Try this one.....

​Percent composition lego activity (pg 27)

​Percent composition lego activity (pg 27)

Each group will receive 10 lego pieces

• 3 will be given to you

• 7 you may choose

• On the worksheet draw a rough picture or description of each DIFFERENT shaped piece you have (color does not matter, if the pieces are the same size and shape, they count as the same piece)

• Use the balance to get the mass of each different piece on your list

• Fill in your table with the mass and # of blocks for each different type

• Build a lego structure and calculate the molar mass of your structure 

• Next, calculate the percent composition of each different piece

• Once you have calculated percent comp for each piece you may build something else!

• If you finish earl​y, work on pages 28-29, or 24-25.