Stoichiometry Worksheet (Page 2 of 6) in pdf



The mass of a proton is approx. 1.008 amu. The mass of a neutron is approximately 1.009 amu. The mass

of the electron is insignificant. When elements are formed from the individual subatomic particles a large

amount of energy in the form of heat is released. This loss of mass is called the mass defect and this

conversion of mass into energy is seen in the equation: E = mc

Gram Formula Mass / Molar Mass



Gram formula mass (also known as molar mass) is defined as the atomic mass of one mole of an element,

molecular compound or ionic compound. The answer must always be written with the unit g/mol (grams

per mole).

Calculating a substance's gram formula mass (molar mass):

Calculate the gram formula mass(molar mass) of ammonium phosphate.

Description of Action

Action

1. Write the formula for the compound.

1. (NH

)

2. Determine how many of each atom are in the

2. N: 3

compound. If there is a number outside the parenthesis,

H:12

multiply each subscript by this number. If there is no

P: 1

subscript, assume it is one.

O:4

3. Multiply the number of atoms by its atomic weight.

3. N: 3 x 14.0 = 42.0

Round the atomic weight to the tenth’s place.

H:12 x 1.0 = 12.0

P: 1 x 31.0 = 31.0 (31.0 is rounded from 30.97)

O:4 x 16.0 = 64.0 (16.0 is rounded from 15.99)

4. Add your results and use the unit g/mol (grams per

4. 42.0 + 12.0 + 31.0 + 64.0 = 149.0 g/mol.

mole) on the end.

Mole Conversions (LO 1.4)



To convert between moles, atoms, molecules and liters you should use

the following chart:



This topic and chart are covered extensively in first year chemistry.

For more practice/information check out the link to my

Honors/Chemistry I worksheet:



One example is shown below:

Calculate the number of hydroxide ions in 23.5 grams of aluminum hydroxide.

23.5 g Al(OH)

÷ 78.0 g Al(OH)

x 6.022 x 10

x 3 = 5.44 x 10

(OH)

Percent Composition (LO 1.3)



When we calculate percent composition, we are determining the relative mass that each element contributes

to the total mass of the compound. For example, if we calculate the gram formula mass of H

O we will

find it to be 18.0 g/mol. We arrived at this number by adding the mass of oxygen, 16.0, and the mass of

two hydrogens, 2.0. Oxygen makes up 16.0 of the total 18.0 grams. Hydrogen is 2.0 of 18.0 grams. If we

divide each elements total mass by the compounds total mass and multiplying this result by 100, we get a

percentage. This percentage is the element’s percent composition.

Example (from above):

STEP 1

STEP 2

STEP 3

(find g.f.m.)

(divide each element mass by g.f.m.)

(multiply by 100)

H: 2 x 1.0 =

2.0

H: 2.0 ÷ 18.0 = 0.111

H: 0.111 x 100. = 11.1%

O: 1 x 16.0 = 16.0

O: 16.0 ÷ 18.0 = 0.889 (rounded)

O: 0.889 x 100. = 88.9%

Total = 18.0 g/mol

Stoichiometry Worksheet Page 2

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