Stoichiometry Chapter 9

Step 1

• Balance equations and calculate Formula Mass (FM) for each reactant and product.

• Example:

Tin (II) fluoride, SnF2, is used in some toothpastes. It is made by the reaction of tin with hydrogen fluoride, HF, according to the following equation.

Sn (s) + HF(g) SnF2 (s) + H2 (g)

How many grams of SnF2 are produced from the reaction of 30.00g of HF with Sn?

Step 1

• Balance the equation • Sn (s) + HF(g) SnF2 (s) + H2 (g)

• Sn (s) + 2HF(g) SnF2 (s) + H2 (g)• Calculate Formula Mass for all reactants and

products.• Sn = 119 g/mol HF= 1+ 19= 20 g/mol

SnF2= 119 + (2x19) = 157 g/mol H2= 2x1= 2 g/mol

Step 2

• In the example problem we were given mass.–How many grams of SnF2 are produced from

the reaction of 30 g of HF with Sn?–We must turn the grams of HF into moles of

HF.• Example:

• 30gHF 1molHF 20gHF

Step 3

• Determine the ratio of moles from the balanced equation.

Sn (s) + 2HF(g) SnF2 (s) + H2 (g)

• The ratio is 1mol SnF2 : 2 mol HF

30gHF 1molHF 1molSnF2 2molHF 20gHF

Step 3

• BEWARE:–If 2 or more givens are listed, you must

determine the limiting reagent: this is the reactant that you have the least amount of moles.–ALWAYS use the limiting reagent to

calculate the problem. If you do not, your answer will ALWAYS be WRONG.

Step 4

• After calculating the mole ratios, proceed to calculate the mass for the unknown. If question only asks for moles of unknown, you are finished.

• Example:–How many grams of SnF2 are produced from

the reaction of 30 g of HF with Sn?

20gHF 2molHF30gHF 1molHF 1molSnF2 157g SnF2=

1molSnF2 117.75g SnF2

How To Calculate Percent Yield

• You would follow all of the steps for solving stoichiometry .

• Using the example from above, let say that the problem read:–When 30.00 grams of HF reacts with an

excess of Sn, the actual yield of SnF2 is 100.5 g. What is the percent yield?

How To Calculate Percent Yield

• You would have followed all of the steps to calculate that the theoretical yield of SnF2 was 117.75g.

•Percent yield = actual yield x 100 theoretical yield

Percent yield= 100.5 g x 100 = 85.3% 117.75g