S

Percentage Composition

SCH3U

In a group, calculate the percentage of flasks, owls,

Marios, pencils & stars

What if this was related to mass? Calculate the % by mass for each of the objects.

100 g

20 g

50 g

2 g

6 g

Oreos

Obtain an Oreo. The cookie is made up of WAFER and CREAMY FILLING. Using a balance, a plastic knife and a napkin (to keep the cookie from directly touching the balance), calculate the percent of the cookie that is made up of each part. You have 10 minutes to complete and be ready to explain the process and your answer.

% of Creamy Filling

Regular Oreo Cookie Double Stuffed Oreo

1

2

3

4

5

6

Fruit

Obtain a small slice of fruit. Find the percent of water in the piece of fruit.

What do you have to do first? Mass out your sample using a balance You will have to leave the sample in the drying oven

overnight at 200°F.

Percentage Composition in YOUR life

Centium Lab…Metal Composition in Coins

Scientists use the percentage composition of natural substances to identify the chemical formula of a compounds. Once the chemists know the chemical formula, they can often produce the compound in a lab…enjoy your artificial vanilla!

Law of Definite Proportions

Elements in a chemical compounds are always present in the same proportions by mass Eg. For water: 2 H for every 1 O

Percentage Composition

The relative mass of each element in a compound is called Percentage Composition. Mass Percent is the mass of an element in a compound

Expressed as a percentage of the total mass of the compound

Eg. Vanillin, C8H8O3, has: Mass percent of C = 63.1% Mass percent of H = 5.3% Mass percent of O = 31.6%

Total: 100%

a percentage composition of 63.1% C, 5.3% H and 31.6% O.

Methods for Calculating Percentage Composition

There are two ways to calculate the mass % of each element in a compound:

1. Using the mass of the compound and each element mass % = mass of the element x 100%

mass of the compound

2. Using the chemical formula of the compound:mass % = (# of the element in cpd) x

MMelement x 100%MMcompound

Use when you

know mass

Use when you know

the chemical formula

Example 1

A sample of a compound is analyzed and found to contain carbon, hydrogen and oxygen. What is the percent composition of the compound?

msample = 650 mg; mC = 257 mg; mH = 50.4 mg

mO = msample – mC - mH = 343 mg

Mass % C = 257 mg/650mg x 100%Mass % H = 50.4 mg/650 mg x 100%Mass % O = 343 mg/650mg x 100%

Therefore the percentage composition of the compound is 39.5% C, 7.75% H, 52.7% O.

We know the masses

Example 2

Sulfuric acid is an important acid in labs and industries. Determine the percentage composition of sulfuric acid.

MMH2SO4 = 98.09 g/mol

Mass % H = 2 x 1.01 g mol-1 /98.09 g mol-1 x 100%Mass % S = 1 x 32.06 g mol-1 /98.09 g mol-1 x 100%Mass % O = 4 x 16.00 g mol-1 /98.09 g mol-1 x 100%

Therefore the percentage composition of sulfuric acid is 32.69% S, 2.06% H, 65.24% O.

We know the chemical formula

Fruit Cont…

% of water in fruitBanana Grapes Apple Oranges

1

2

3

4

5

6

Try These…

Activity p260 textbook – Mass Percent of Oxygen

p267 #4, 6, 10, 12, 13

Hydrate Lab

Obtain 1.5 to 2.5 grams of hydrate. Record your data in a table

Find the percent of water present in the sample. You may use any items present on the lab counters.

Calculations and questions Calculate the number of moles of water lost (nwater). Calculate the number of moles of anhydrous salt (nanhydrous salt) Calculate the number of moles of water lost for every mole of

anhydrous salt (mol water ÷ mol anhydrous salt). The above calculation gives the value of x in your hydrate

[CuSO4·xH2O(s)]. Write the correct formula for the hydrate.

The objective of this experiment is to determine the number of moles of water associated with one

mole of a hydrate.