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Stoichiometry

Stoichiometry is the mathematical method for converting from one substance to another.

The important new step is determining the molar ratio.

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Molar Ratio

The molar ratio is based on the balanced equation.

Reading the balancing equation, the coefficients can be read as moles instead of just numbers.

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Balancing Equations

NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Balanced becomes:

2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Reading the equation we can say 2 moles of sodium bicarbonate react to form 1 mole sodium carbonate, one mole water and one mole carbon dioxide.

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Molar Ratio

The molar ratio can be found by comparing the coefficients.
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

2 moles NaHCO₃ : 1 mole Na₂CO₃
2 moles NaHCO₃ : 1 mole H₂O
2 moles NaHCO₃ : 1 mole CO₂

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UNITS!

Units units units units units units units units units unit units.
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Units units units.
Units units units units units units units units units units units units units units units units units units units units.

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Units

Units will be very important in stoichiometry calculations.
Units will guide you from one set of information to the other.
Using the same arrangement from unit conversions will help organize the problems.

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Molar Ratio

Sometimes problems are only interested in how many moles of a substance are needed to reach another.

To solve we only need the molar ratios from the balanced equation.

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Chlorine is used by textile manufacturers to bleach cloth. Excess chlorine is destroyed by its reaction with sodium thiosulfate, Na₂S₂O₃:

__Na₂S₂O₃+ __Cl₂+ __H₂O → __NaHSO₄+ __HCl

a. How many moles of Na₂S₂O₃ are needed to react with 0.12mol of Cl₂?

b. How many moles of HCl can form from 0.12mol of Cl₂?

c. How many moles of H₂O are required for the reaction of 0.12mol of Cl₂?

d. How many moles of H₂O react if 0.24mol HCl is formed?

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Stoichiometry

Using this ratio we can calculate the theoretical values of how much of each product would be produced.

If 2.33 g of sodium bicarbonate was decomposed to produce sodium carbonate, carbon dioxide and water. What are the masses of each product?

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Stoichiometry

If 2.33 g of sodium bicarbonate was decomposed to produce sodium carbonate, carbon dioxide and water. What are the masses of each product?
Starting with the given over 1, we set up the equation the same as a unit conversion question.

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Stoichiometry

If 2.33 g of sodium bicarbonate was decomposed to produce sodium carbonate, carbon dioxide and water. What are the masses of each product?
To solve for water:

2.33 g NaHCO₃	1 mole NaHCO₃	1 mole H₂O	18.01 g H₂O
1	84.01 g NaHCO₃	2 mole NaHCO₃	1 mole H₂O

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Limiting Reagent

When a chemical reaction occurs, we rarely use up all reactants. When one reactant is more than enough it is said to be in excess.
When a reactant runs out first it is said to limit the amount produced, or be called the limiting reagent.

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Limiting Reagent

To determine the limiting reagent you need to calculate the products made for each of the reactants stoichiometrically. Whichever reactant makes less product is limiting.
A reaction occurs between 3.0 g silver nitrate and 2.5 g sodium chloride to produce silver chloride. How much silver chloride is produced? Which is the limiting reagent?

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Limiting Reagent

Start by balancing the equation.
Next, calculate how much product would be made from each of the starting components.
3.0 g AgNO₃

2.5 g NaCl

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Percent Yield

Percent yield is a calculation to compare theoretical and actual yields.
Theoretical yields are calculated from what we started with and are ‘expected’ values.
Actual yields come from experiments and are what were ‘actually’ produced.

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Percent Yield

Percent Yield = Actual x 100 %

Theoretical

A reaction produced 2.33 g of CO₂. When doing the stoichiometry, students calculated 3.23 g of CO₂ should be produced. What is the percent yield for the reaction?