Warmup

A fruit salad recipe calls for 2 oz of diced cherries per 4 oz sliced apple and 3 oz pulped oranges.

A chef has 10 oz of cherries, 24 oz apples, and 18 oz oranges.

1) Which type of fruit will run out first?

2) Based on the amount of fruit available, how many ounces of finished fruit salad should be created? (Assume the recipe is followed and some fruit doesn’t get completely used)

3) If 41 oz of fruit salad is produced, what percentage of the anticipated total was yielded?

Limiting Reagent and Percent Yield

November 28, 2011

Definitions

Limiting Reagent – the reactant which runs out first and limits how much can be produced

Percent Yield – measures the fraction of the total anticipated among the products of a reaction.

How-to

1) Balance the equation. The coefficients express the mole ratio of the reactants and products to each other.

Ex: A fruit salad recipe calls for 2 oz of diced cherries per 4 oz sliced apple and 3 oz pulped oranges.

Ex: 1CH4 + 2O2 1CO2 + 2H2O

“For every 1 mole of Methane you’ll get 1 mole of carbon dioxide.” “For every one mole of water you’ll need ½ a mole of methane.”“For every 1 mole of oxygen, you’ll get one mole of water.”

How-to

2) From a word problem, convert grams to moles using the molecular mass of each molecule.

Ex: I have 30g of CH4 and 10g of H2O.

For CH4, (12+4*1)g/mol

For H2O, (2*1+16)g/mol

1 mol/16g * 30g/1 = 1.88 moles of methane 1 mol/18g * 10g/1 = 0.56 moles of oxygen

How-to

3) Divide the number of moles by the coefficients from the chemical equation, and identify the reactant you have the least of. This is the limiting reagent.

1CH4 + 2O2 1CO2 + 2H2O,

1.88 moles of methane, 0.56 moles of oxygen

1.88/1 for methane, 0.56/2=0.28 for oxygen.Oxygen is the limiting reagent.

How-to

4) Identify how many moles of each product you expect. Use proportions.

1CH4 + 2O2 1CO2 + 2H2O,

0.28 moles:2 (oxygen and water)0.14 moles:1 (methane and carbon dioxide)

How-to

5) Theoretical Yield* Percent = Actual Yield

Use that idea to solve for the missing variable. Convert back to grams if necessary.

Ex: The reaction has a yield of 80%. How much carbon dioxide is produced?

0.8 * 0.14 moles = 0.11 moles CO2

16 g/1 mol * 0.11 mol/1 = 1.76 g

Practice Problem

54g of aluminum metal reacts with 32g of oxygen to form 51g of aluminum oxide.

What is the limiting reagent? What is the percent yield?

Practice Problem

100g of nitrogen trichloride reacts with an excess of fluorine gas. If the reaction has a 75% yield, what is the limiting reactant and how many grams of fluorine are consumed?