Aim: How to calculate Percent

Composition

DO NOW:

1. What is the number of moles of

potassium chloride present in 148 g?

2. What is the molar mass of CuSO4 5H2O?

3. What is the mass of 3.01 x 1023 atoms of

carbon?

Percent Composition

The percent composition is the mass percentage

of each type of atom (element) in a compound.

% mass of element = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑x 100%

Percent Composition from Mass

Data Percent composition can be calculated from experimental data.

A student prepares a compound of tungsten chloride from 3.946 g of tungsten and 3.806 g of chlorine. Calculate the percent composition.

% W = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑊

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑊𝐶𝑙2x 100% =

3.946 𝑔

7.752x 100% = 50.9 %

% Cl = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝐶𝑙

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑊𝐶𝑙2x 100% =

3.806 𝑔

7.752x 100% = 49.1 %

Percent Composition from

Chemical Formula

% mass = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 1𝑚𝑜𝑙 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑

𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑x 100%

Percent Composition from

Chemical FormulaTo calculate the percent composition of a component in a compound:

1. Find the molar mass of the compound by adding up the

masses of each atom in the compound.

2. Calculate the molar mass of the component in the

compound for which you are solving.

3. Divide the molar mass of the component by the total

molar mass of the compound and multiply by 100

Percent Composition from

Chemical Formula

Step1

8C + 10H +4N + 2O

(8 x 12.0 g/mol) + (10 x 1.0 g/mol) + ( 4 x

14.0 g/mol) + (2 x 16.0 g/mol) = 194.0

g/mol

Step 2

Carbon: 8 𝑥 12.0 𝑔/𝑚𝑜𝑙

194 𝑔/𝑚𝑜𝑙x 100% = 49.5 %

Hydrogen: 10 𝑥 1.0 𝑔/𝑚𝑜𝑙

194 𝑔/𝑚𝑜𝑙x 100% =5.1 %

Nitrogen: 4 𝑥 14.0 𝑔/𝑚𝑜𝑙

194 𝑔/𝑚𝑜𝑙x 100% = 28.9 %

Oxygen: 100% - (49.5 + 5.1 + 28.9) = 16.5 %

Percent Composition of Crystal

Hydrates

A HYDRATE is a CRYSTALLINE compound in which

ions are attached to one or more WATER molecules.

Example: Na2CO3 10H2O

Notice how WATER molecules are BUILT INTO the

chemical formula

Substances without water built into the formula are

called ANHYDRATES.

Problem: What is the percentage by

mass of water in sodium carbonate

crystals (Na2CO3 10H2O)?Step 1: Calculate the formula mass for the hydrate.

Na = 2 x 23.0 g/ mol = 46. 0 g/mol

C = 1 x 12.0 g/mol = 12.0 g/mol

O = 3 x 16. 0 g/mol = 48.0 g/mol

H2O = 10 x 18.0 g/mol = 180 g/mol

Formula mass of hydrate = 286 g/mol

Step 2: Set up and Calculate

% composition by mass = mass of part/ mass of whole X 100%

% H2O by mass =(180 g/mol)/(286 g/mol)

x 100%

% H2O by mass = 62.9 %

Percent Composition as a

conversion Factor http://wps.prenhall.com/wps/media/objects/477/488597/le0602.html

We can use mass percent composition as a

conversion factor between grams of an element

and grams of the compound.

The mass percent composition of sodium in sodium

chloride is 39%.

This can be written as:

39 g of Sodium : 100 g of Sodium Chloride

This can be written in fractional form:

39 𝑔 𝑁𝑎

100 𝑔 𝑁𝑎𝐶𝑙or

100 𝑔 𝑁𝑎𝐶𝑙

39 𝑔 𝑁𝑎

These fractions are conversion factors between

grams of Na and grams of NaCl.

The FDA recommends that adults consume less

than 2.4 g of sodium per day. How many grams

of sodium chloride can you consume and still be

within the FDA guidelines? Sodium chloride is 39%

sodium by mass.

Known Unknown

2.4 g Na grams of NaCl

39 g Na : 100 g NaCl

2.4 g Na x 100 𝑔 𝑁𝑎𝐶𝑙

39 𝑔 𝑁𝑎= 6.2 g NaCl

Practice Problems

1. A 45.9 g sample of copper, Cu, combined

with oxygen to give 51.69 g of an oxide compound. The percent composition of this compound is:

a. 88.8 % Cu, 11.2% O

b. 24.4% Cu, 75.6% O

c. 60.1% Cu, 39.9 % O

d. 11.3% Cu, 88.7% O

2. Calculate the percent compost ion of

each set:

H2O

H=___11.1%_____ O=______88.9%______

H2O2

H = ____5.88%____ O =_______94.1%_____

3. How many grams of sodium will

combine with 567.0 g of Sulfur from

Na2S? (Hint: use % composition as

conversion factor)

815.6 g