Chapter 5: Unit 7. Determination of mass and Moles of a Chemical Reaction

Determination of mass and Moles of a Chemical Reaction

Stoichiometry is a big word for a process that chemist’s use to calculate amounts in reactions. It makes use of the coefficient ratio set up by balanced reaction equations to make connections between the reactants and products in reactions.

Stoichiometry calculates the quantities of reactants and products in a chemical reaction. The equation must be balanced!

Questions that deal with amounts in reactions are examples of reaction stoichiometry. We already have the tools necessary to solve this question. We just need to learn a new way to apply skills such as writing chemical formulas, calculating formula masses, and converting from mass to moles, particles to moles, and volume of gases to moles.

Mole to Mole Conversions- you must have the Mole Ratio. Illustration: Let’s use an analogy that we can understand to begin to understand the process. The KEY to any mole conversion is the ratio of coefficients in the reaction equation.

Say I want to make a bacon double cheeseburger. Let’s get our recipe together. 1 hamburger bun + 2 hamburger patties + 2 slices of cheese + 4 strips of bacon → 1 bacon double cheeseburger

Based on this recipe: 1) If I have five bacon double cheeseburgers:

a) How many hamburger buns did you use? → 5 hamburger bun

b) How many hamburger patties did you use? → 10 hamburger patties

c) How many slices of cheese did you use? → 10 slices of cheese

d) How many strips of bacon did you use? → 20 strips of bacon

2) How many bacon double cheeseburgers can you make if you start with:

a) 2 buns, 4 patties, 4 slices of cheese, 8 strips of bacon ans: 2 bacon double cheese burgers

b) 1 dozen buns, 2 dozen patties, 2 dozen slices of cheese, 4 dozen strips of bacon ans: 1 dozen bacon double cheese burgers 2

c) 1 mole of buns, 2 mol of patties, 2 mol of cheese slices, 4 mol of bacon strips ans: 1 mole of bacon double cheese burgers

d) 10 buns, 20 patties, 2 slices of cheese, 40 strips of bacon, ans: only 1 bacon double cheese burger

To think through these questions we were using the ratios set up by the reaction equation (a.k.a. recipe) We understood that to produce one complete bacon double cheeseburger we needed to have each of the above ingredients in a 1 bun to 2 patties to 2 slices of cheese to 4 slices of bacon. This ratio is called the coefficient ratio. The coefficient of a balanced chemical equation tell us the number of moles of each reactant that combine and the number of moles of each product formed. Coefficients are used to form molar ratios that serve as conversion factors relating the number of moles of reactants and products.

When the mass of a substance in a reaction must be calculated, first its number of moles is determined using mole rations, and then the molar mass is used to convert moles to grams.

If we want to make 3 sandwiches then we would just triple all of the coefficients. THIS COEFFICIENT RATIO IS KEY TO DOING STOICHIOMETRY.

Let’s apply the above logic to a chemical recipe (a.k.a. chemical reaction equation)

1. : 1 H₂SO₄ + 2 NaOH → 1 Na₂SO₄ + 2 H₂O

If I use 1 mole of H₂SO₄ How many moles of sodium hydroxide do I need? 2 moles of NaOH

2How many moles of sodium sulfate do I make? 1 Mole of Na₂SO

2. Consider the reaction 2H₂ + O₂ → 2H₂ How many moles of water will be produced if there are 3.5 moles of oxygen?

pathway: mol of O₂ → 2 mol of H₂O

3.5 moles O_{2 *} 2 moles H₂O = 7.0 mol H₂O

3. Mole to Mass.

How many grams of water will be produced if there are 3.5 moles of oxygen?

pathway: mol of O₂ → mol of H₂O → grams of H₂O

3.5 mol O₂ → 2 mol H₂O   =  7.0 mols H₂O

1mol O₂

We need to mols to grams of H₂O.Here molar mass is the conversion factor.

18.02 g H₂O =  1mol H₂O

mols of H₂O *  02 g H₂O    = 126 g of H₂O or 130 g of H₂O.

1 mol H₂O

4. Mass to Mass. What mass of barium phosphate can be produced from 14.3 g of potassium phosphate reacting with barium nitrate?

2K₃PO₄ + 3Ba(NO₃)₂ → Ba₃(PO₄)₂ + 6 KNO₃

pathway: grams of K₃PO₄ → mol of K₃PO₄ → mol of Ba₃(PO₄)₂ → grams of Ba₃(PO₄)₂

14.3 g K₃PO₄ ×1 mol K₃PO₄ ×1 mol Ba₃(PO₄)₂   ×       601.93 g Ba₃(PO₄)₂          =  20.3 g Ba3(PO4)2

212.27 g K3PO4           2 mols    K3PO4            1 mol Ba₃(PO₄)₂

Molar mass of K3PO4= 212.27 g/mol

Molar mass of Ba3(PO4)2= 601.93 g/mol

This is the summary of stoichiometry:

Here is another video explaining how to solve step by step stoichiometry problems.

https://youtube.com/watch?v=lcnIiC8JZg0%3Fstart%3D80%26feature%3Doembed

Following Activity has been taken from AACT

Air Bag Stoichiometry

Background

• Stoichiometry

• mass relationships between substances in a chemical reaction
• based on the mole ratio
• Mole Ratio
• indicated by coefficients in a balanced equation

Prelab Questions

1. Have you ever had an experience with a vehicular air bag?  Do you know someone who has?

1. Why would an air bag need to be inflated with an exact amount of gas?

1. What might happen if an air bag was inflated with too little gas?

1. What might happen if an air bag was inflated with too much gas?

Materials

• Calculator
• Periodic table

Problem

If exactly 59.6g of nitrogen gas is needed to inflate your air bag to the correct size, how many grams of NaN₃ would you need to decompose? (Chemical reaction must be written and balanced first)

Extension

The compound diborane (B₂H₆) was at one time considered for use as a rocket fuel.  How many grams of liquid oxygen would a rocket have to carry to burn 10 kg of diborane completely?  (The products are B₂O₃ and H₂O).

Individual practice problems

1. You want to help your little brother make an exploding volcano for his science class. The lava will be made from reacting baking soda (NaHCO₃) with vinegar (HC₂H₃O₂).  After building the volcano, you know that you want to create about 100.0g of lava (or sodium acetate).  Too little lava, and the volcano won’t overflow.  Too much lava would be a giant mess!  Using stoichiometry and the equation below, calculate the exact amount of baking soda needed to make 100.0g of lava. Assume you have excess vinegar.

NaHCO₃ + HC₂H₃O₂ → NaC₂H₃O₂ + H₂O + CO₂

2. Camels store the fat tristearin (C₅₇H₁₁₀O₆) in the hump. As well as being a source of energy, the fat is a source of water, because when it is used the reaction below takes place. What mass of water can be made from 1.0kg of fat?

2 C₅₇H₁₁₀O₆  +  163 O₂  à  114 CO₂  +  110 H₂O

3. You want to create 12g of copper to meld into a piece of jewelry. You know that when copper (II) chloride reacts with aluminum, copper is a product. How much aluminum would you need to start your reaction with to get 12g of copper? (write and balance the reaction first)

Individual practice problems – Answers

1. You want to help your little brother make an exploding volcano for his science class. The lava will be made from reacting baking soda (NaHCO₃) with vinegar (HC₂H₃O₂).  After building the volcano, you know that you want to create about 100.0g of lava (or sodium acetate).  Too little lava, and the volcano won’t overflow.  Too much lava would be a giant mess!  Using stoichiometry and the equation below, calculate the exact amount of baking soda needed to make 100.0g of lava. Assume you have excess vinegar.

NaHCO₃ + HC₂H₃O₂ → NaC₂H₃O₂ + H₂O + CO₂

= 102.4g NaHCO3

100.0gNaC₂H­₃O₂       1 mol NaC₂H­₃O₂        1 mol NaHCO_3­­           84.01g NaHCO₃

82.04g NaC₂H­₃O₂      1 mol NaC₂H­₃O₂        1 mol NaHCO₃

2. Camels store the fat tristearin (C₅₇H₁₁₀O₆) in the hump. As well as being a source of energy, the fat is a source of water, because when it is used the reaction below takes place. What mass of water can be made from 1.0kg of fat?

2 C₅₇H₁₁₀O₆  +  163 O₂  à  114 CO₂  +  110 H₂O

1.0kg C₅₇H_110­O₆         1000g    1 mol C₅₇H_110­O₆          110 mol H₂O        18.02g H₂O

1 kg        891.67g C₅₇H_110­O₆  2 mol C₅₇H_110­O₆    1 mol H₂O

= 1.1 x 103 g H2O

3. You want to create 12.0g of copper to meld into a piece of jewelry. You know that when copper (II) chloride reacts with aluminum, copper is a product. Given excess copper (II) chloride, how much aluminum would you need to start your reaction with to get 12.0g of copper? (write and balance the reaction first)

2Al  + 3CuCl₂  à 2AlCl₃  +  3Cu

= 3.40g Al

12.0g Cu         1 mol Cu         2 mol Al          26.98g Al

63.55g Cu       3 mol Cu         1 mol Al

Questions:

1. What mass of bromine gas can be produced from the complete reaction of potassium bromide and 34.5 g of fluorine gas? 2KBr + F₂ → Br₂  + 2KF

2.      Lime, CaO, is produced by the reaction CaCO3(s) → CaO(s) + CO2(g). What weight of CO2 is obtained by the decomposition of 38.7 g of CaCO3?

3.      Given the reaction 2 HgO(s) → 2 Hg(l) + O2(g). What weight of elemental mercury will be obtained by the decomposition of 94.5 g of HgO?

Ans: 1. 145 g Br21.      17.0 g CO22.      87.5 g Hg