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CHAPTER 9. Chemical Quantities. 9.1.

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### CHAPTER 9

Chemical Quantities

9.1

- -The equation for a chemical reaction indicates the relative numbers of reactant and product molecules required for the reaction to take place. Using the equation permits us to determine the amounts of reactants needed to give a certain amount of product or to predict how much product we can make from a given quantity of reactants.
- a. For example a box of cake mix gives ingredients and instructions to make a certain amount of cake batter.

9.1 (continued)

- Now we can use an equation to predict moles
- The moles of products can be predicted by the given number of moles of reactants will yield.
- For example: The decomposition of 2 mol of water = 2H2O – 2H2 + O2
- This tells us that the water yields 2 mol of H2 and 1 mol of O2.

9.1: More Practice

- How much is yielded from the decomposition of 4 mol of water?
- 4 mol of H2 and 2 mol of O2
- Since the total value was just multiplied by 2, we multiply the reactants by 2 as well.

9.1: Using Mole Ratios

- If the number isn’t easily exchangeable, as 2 and 4 are, then we have to balance the equation first.
- For example: What if 5.8 mol of water is decomposed?
- First we balance the equation by dividing by two, resulting in H2O – H2 + ½ O2

- Then multiply each by 5.8, resulting in 5.8H2O – 5.8H2 + 2.9O2.

9.2

- Section is talking about balancing equations by getting the same weights. You change the coefficients.
- You multiply the mass in grams X the value of one mole of that element or substance over the molar mass of the substance in grams

9.2 continued

- 1. balance the equation for the reaction
- 2. convert the masses of reactants of products to moles
- 3. use the balanced equation to set up the appropriate mole reactions
- 4. use mole ratio(s) to calculate the number of moles of the desired reactant or product
- 5. convert back to mass
- Example:

9.2. continued

- Stoichiometry- the process of using a balanced chemical equation to determine the relative masses of reactants and products involved in a reaction

9.2 continued

- When you compare two reactions, they need to be balanced, and you would be comparing how much of a same substance they will react with.
- Example: two antacids: baking soda and milk of magnesia both consume stomach acid, but in order to find out which consumes more, you need to find how many moles of stomach acid reacts with a gram of each.

9.2 practice

- We need to know how much carbon dioxide 1.00x10^3 lithium hydroxide can absorb. The products are water and lithium carbonate
- The equation would be:
LiOH(s) + CO2(g) → Li2CO3(s) + H2O(l)

- What is the balanced equation?

More practice

- 2LiOH(s) + CO2(g) → Li2CO3(s) + H20(l)
- Convert the given mass of LiOH into moles using the molar mass: 23.95g

- 41.8 mol L
- The mole ration is 1 mol CO2
2 mol LiOH

- Thus calculate the moles of CO2

- 20.9 mol CO2
- Calculate the mass of CO2 using its molar mass: 44.01 g

- 920g.
- Therefore, 1.00x10^3 g of LiOH(s) can absorb 920 g of CO2(g)

9.3 Limiting Reactants and Percent Yield

- Limiting Reactant- The reactant in a chemical reaction that limits the amount of product that can be formed.

9.3 The Concept of limiting reactants

- If you are making a chain reaction, you need to form a ratio with all of the reactants.
- All ingredients must be used in the correct ratio

9.3 (continued)

- The reactant that runs out first is the limiting reactant. You can not continue to produce that product with an item after it has run out.

9.3 Practice

- 2 slices of bread + 3 pieces of meat + 1 slice of cheese = 1 sandwich
- To make 10 sandwiches, you would have to multiply 10 to each of the ingredients.
- 2(10) + 3(10) + 1(10) = 10 sandwiches
- 20 slices of bread, 30 pieces of meat, and 10 slices of cheese will make 10 sandwiches.

9.3 More Practice

- 2 slices of bread + 3 pieces of meat + 1 slice of cheese = 1 sandwich
How much of each item would I need to make 40 sandwiches for the football team at their upcoming game?

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