Percent CompositionPercent Composition

This tutorial will help you learn This tutorial will help you learn how to calculate the percentage how to calculate the percentage

by mass of elements in a by mass of elements in a compoundcompound

What is the percentage of oxygen What is the percentage of oxygen in Potassium chlorate?in Potassium chlorate?

Place 1.29g of KClOPlace 1.29g of KClO33 in a crucible in a crucible

and heat it for at least 10 minutes. and heat it for at least 10 minutes. Heating this compound will drive the Heating this compound will drive the oxygen atoms out of the compound oxygen atoms out of the compound and into the air as oxygen gas.and into the air as oxygen gas.

After heating, allow the crucible to After heating, allow the crucible to cool to room temperature.cool to room temperature.

The mass of the contents in the crucible is now 0.78g. All of the oxygen has been driven off and only KCl remains in the crucible. What is the mass of oxygen that was driven off?

CORRECTCORRECT1.29g – 0.78g = 0.51g of oxygen1.29g – 0.78g = 0.51g of oxygen

Now determine the percentage of Now determine the percentage of oxygen in the compound by taking oxygen in the compound by taking the mass of they oxygen divided by the mass of they oxygen divided by the mass of the compound and the mass of the compound and multiplying by 100. Show your work multiplying by 100. Show your work on your paper.on your paper.

GOOD WORKGOOD WORK(0.51g/1.29g)x 100 = 39.1% oxygen(0.51g/1.29g)x 100 = 39.1% oxygen

Let’s now do some problems when Let’s now do some problems when we only know the formula of the we only know the formula of the compound.compound.

What is the mass of carbon in What is the mass of carbon in carbon dioxide?carbon dioxide?

#1 – Find the mass of the atom. #1 – Find the mass of the atom. Carbon has a mass ofCarbon has a mass of 12.0g12.0g

#2 – Find the mass of the compound#2 – Find the mass of the compound

COCO22 has a molecular mass of 44.0g. has a molecular mass of 44.0g.

#3 – Mass of atom/mass compound #3 – Mass of atom/mass compound and then multiply by 100. and then multiply by 100.

(12.0/44.0) x 100 = 27.3% carbon(12.0/44.0) x 100 = 27.3% carbon

Practice problemsPractice problemsWhat is the percentage by mass of iron What is the percentage by mass of iron

in Iron(III) chloride?in Iron(III) chloride?

Iron(III) chloride is FeCl3.

Iron has a mass of 55.8g

FeCl3 has a molecular mass of 162.3

(55.8/162.3) x 100 = 34.4%

Practice problemsPractice problemsWhat is the percentage by mass of What is the percentage by mass of oxygen in carbon monoxide?oxygen in carbon monoxide?

Carbon monoxide is CO.Carbon monoxide is CO.

Oxygen has a mass of 16.0g.Oxygen has a mass of 16.0g.

CO has a molecular mass of 28.0gCO has a molecular mass of 28.0g

(16.0/28.0) x 100 = 57.1%(16.0/28.0) x 100 = 57.1%

Practice problemsPractice problemsWhat is the percentage by mass of What is the percentage by mass of chlorine in magnesium chloride?chlorine in magnesium chloride?

Magnesium chloride is MgClMagnesium chloride is MgCl22..

The mass of chlorine is 35.5g, so two The mass of chlorine is 35.5g, so two chlorines will have a mass of 71.0g.chlorines will have a mass of 71.0g.

The mass of MgClThe mass of MgCl22 is 95.3g. is 95.3g.

(71.0/95.3) x 100 = 74.5% (71.0/95.3) x 100 = 74.5%