## Mass-Mass Calculation Example One: Potassium Nitride Decomposition

How many grams of potassium nitride, KN(s), can be consumed when 20.1g of nitrogen gas, N(g), is produced?

The first step is to convert nitrogen’s gram quantity to moles of nitrogen gas, N(g). The second step uses the stoichiometry of the balanced equation

2KN(s)  1N(g) + 6K(s)

to find the moles of potassium nitride, KN(s), consumed. In the third step the molar amount of potassium nitride, KN(s),  is then converted to grams of potassium nitride, KN(s).

Step 1: One mole of nitrogen gas, N(g), is 28.02 grams/mole and is found using the gram atomic mass(14.01g) of nitrogen from the periodic table. The formula mass calculation is as follows.

Element   Subscript   X   Element Mass = Formula Mass

N                2           X      14.01g           =   28.02g/m

Dividing the given 20.1 grams nitrogen gas, N(g), by the 28.02 grams/mole for the nitrogen gas, N(g), produces moles of nitrogen, N(g), gas formed during reaction.

20.1 N(g) ÷ (28.02g N(g)/1 m N(g)) = 0.717m N(g)

20.1 grams nitrogen gas, N(g), is equal to 0.717 moles of nitrogen gas, N(g).

Step 2: The 0.717m moles of nitrogen gas, N(g), must be converted to moles potassium nitride, KN(s), using the balanced equation

2KN(s)  1N(g) + 6K(s)

0.717m N(g) X (2m KN (s)/1mN(g) = 1.43m KN(s)

Step 3: The 1.43 m potassium nitride, KN(s), must be converted to grams. The formula mass for potassium nitride, KN(s),  is found by the following formula mass calculation using gram atomic masses for each potassium(39.10g) and nitrogen(14.01g) from the periodic table.

Element   Subscript    X   Element Mass = Formula Mass

K                 3           X       39.10g          =    117.30g

N                 1           X       14.01g          =    14.01g

The molar mass of potassium nitride, KN(s), is 117.30g + 14.01g = 131.31g/m.

1.43m KN (s) X (131.31g KN (s)/1m KN (s)) = 188g KN (s)

Therefore, 188g  potassium nitride, KN(s), is produced at the same time 20.1g of nitrogen gas, N(g),  is produced.