Percent Composition

Topic 4.8

Percent Composition

What portion of the overall mass of something is a given part?

Image on the right describes the percent compositions for specific elements of the human body...

The human body is 65% oxygen atoms by mass and only 3.3 % nitrogen atoms by mass.

(consider that we are mostly water)

Percent Compositions in terms of mass.

(i.e. what portion of the overall mass is each element)

Definition of Percent Composition:

The percent by mass of a chemical formula that is a specific element (or component) of that formula.

• Example 1: Oxygen composes 88.8% of H₂O.

• Example 2: Chlorine composes 60.6% of NaCl

• Example 3: Water is 36.1% of CuSO₄ · 5H₂O

Example 1 in detail

The formula mass of  $H_2O$   is 18.02 g/mol.

The contribution to that total from oxygen is 16.00 g/mol.

 $\frac{16.00\ \frac{g}{mol}\ O\ in\ H_2O}{18.02\ \frac{g}{mol}total\ in\ H_2O}\ x\ 100\%\ =\ 88.8\ \%\ Oxygen$  

Example 2 in detail

The formula mass of NaCl is 58.44 g/mol.

The contribution to that total from chlorine is 35.45 g/mol.  (as chloride ion)

 $\frac{35.45\ \frac{g}{mol}\ Cl\ in\ NaCl}{58.44\ \frac{g}{mol}total\ in\ NaCl}\ x\ 100\%\ =\ 60.6\%\ Chlorine$  

Example 3 in detail

The formula mass of  $CuSO_4·\ 5H_2O$   is 249.72 g/mol.

The contribution to that total from water is 90.10 g/mol.   (NOTE: This type of compound is called a hydrate)

 $\frac{90.10\ \frac{g}{mol}H_2O}{249.72\ \frac{g}{mol}\ CuSO_4\ ·\ 5H_2O}\ x\ 100\ =\ 36.1\%$  

Percent Composition Formula

 $\%\ comp\ =\frac{Mass\ contribution\ from\ the\ specific\ element}{Total\ mass\ in\ formula}\ x\ 100$  

Perhaps a simpler formula is this. (From back page of your Reference Tables)

"Mass of part" is the mass contribution from the specific element of interest. (but can also be a polyatomic ion or H₂O)

"Mass of whole" is the total mass of the formula (a.k.a... gram formula mass)

What is the percent composition of Cl in calcium chloride,  $CaCl_2\ ?$  

 $\%\ comp\ =\frac{Mass\ contribution\ from\ the\ specific\ element}{Total\ mass\ in\ formula}\ x\ 100$  

 $The\ mass\ contribution\ from\ Cl\ \ is\ \ 2\ x\ 35.45\ g\ =\ 70.9\ g$   $The\ total\ mass\ of\ the\ formula\ is\ 70.9\ g\ +\ 40.08\ g\ =\ 110.98\ g$  

The mass of calcium is 40.08 and there is only 1 represented in the formula  (Next slide has the answer)

What is the percent composition of Cl in calcium chloride,  $CaCl_2\ ?$   63.89 %

 $\%\ comp\ =\frac{Mass\ contribution\ from\ the\ specific\ element}{Total\ mass\ in\ formula}\ x\ 100$  

 $63.89\ \%\ \ =\ \ \frac{70.90\ grams\ from\ Cl}{110.98\ grams\ total\ mass}\ x\ 100$  

The percent composition is simply  $\frac{part}{whole}\ x\ 100\$   .  

The next 6 slides are 3 questions and 3 explanations that will guide you through an example problem.

Multiple Choice

Considering the generic formula for percent composition below, what number would go in the numerator if you were attempting to solve for the percent composition of Hydrogen in  $CH_4$  ?

 $\%\ comp\ =\frac{Mass\ contribution\ from\ the\ specific\ element}{Total\ mass\ in\ formula}\ x\ 100$  

• 16.05 grams

• 4.04 grams

• 12.01 grams

• 48.04 grams

The mass contribution from hydrogen , H, in CH₄ is 4 x 1.01 grams = 4.04 grams

subscript of 4 indicates there are 4 hydrogen atoms in the formula and each one contributes its atomic mass of 1.01 u , just switched into grams.

Multiple Choice

Considering the generic formula for percent composition below, what number would go in the denominator if you were attempting to solve for the percent composition of Hydrogen in  $CH_4$  ?

 $\%\ comp\ =\frac{Mass\ contribution\ from\ the\ specific\ element}{Total\ mass\ in\ formula}\ x\ 100$  

• 16.05 grams

• 4.04 grams

• 12.01 grams

• 48.04 grams

The total mass of the formula goes in the denominator (a.k.a. the gram formula mass, a.k.a. the molar mass)

From hydrogen there is 4.04 grams, from carbon there is 12.01 grams.

The total is 4.04 + 12.01 = 16.05

subscript of 4 indicates there are 4 hydrogen atoms. The lack of subscript after carbon indicates there is only 1 in the formula.

Multiple Choice

Considering the generic formula for percent composition below and the answers to the two previous questions, what is the percent composition of Hydrogen in  $CH_4$  ?

 $\%\ comp\ =\frac{Mass\ contribution\ from\ the\ specific\ element}{Total\ mass\ in\ formula}\ x\ 100$  

• 80 %

• 4.04 %

• 0.25 %

• 25.17 %

• 74.83 %

The percent composition of hydrogen in  $CH_4$   is 25.17 %

 $\%\ comp\ =\frac{Mass\ contribution\ from\ the\ specific\ element}{Total\ mass\ in\ formula}\ x\ 100$  

 $25.17\ \%\ \ =\ \ \frac{4.04\ grams\ from\ H}{16.05\ total\ mass}\ x\ 100$  
 

You must remember to multiply by 100 after doing the division. Also, there are no units such as grams or moles.