Chemistry – Mrs. Ermann
Stoichiometry
“stoy-kee-ah-met-tree”
The quantitative relationships between the amounts of reactants and products in a chemical reaction
MOLAR RATIOS NOTES
(adapted from: olhs.cksd.wednet.edu/)
The coefficients in abalancedchemical equation can be used to determine the RELATIONSHIP between moles of compounds involved in a chemical reaction.
Example: N2(g) + 3 H2(g) --> 2 NH3(g)
1 molecule of nitrogen (N2) reacts with 3 molecules of hydrogen (H2) to form 2 molecules of ammonia (NH3)
OR
1 mole of nitrogen (N2) reacts with 3 moles of hydrogen (H2) to form 2 moles of ammonia (NH3)
The coefficients in abalancedequation can be used to write amolar ratio. Molar ratios are conversion factors that can be used to relate:
1. moles of product formed from a certain number of moles of reactant
2. moles of reactant needed to form a certain number of moles of a product.
3. the number of moles of a particular reactant needed to completely react with a certain number of moles of a second reactant.
For the following reaction:
4 NH3(g) + 5 O2(g) --> 4 NO (g) + 6 H2O (g)
the following molar ratios can be written:
4 moles NH3
5 moles O2
/ 4 moles NH3
4 moles NO
/ 4 moles NH3
6 moles H2O
5 moles O2
4 moles NO
/ 5 moles O2
6 moles H2O
/ 4 moles NO
6 moles H2O
The inverses of each of these molar ratios can also be written.
The above is written (or spoken) in the following manner for reactants (using the first example): "For every 4 moles of ammonia that reacts, 5 moles of oxygen are needed."
Relating products to reactants, we would say, "For every 5 moles of oxygen that reacts, 4 moles of NO will be produced."
Write the indicated mole ratios for each equation:
1) N2 + 3 H2 → 2 NH3
N2 to H2: NH3 to H2:
2) 2 SO2 + O2 → 2 SO3
O2 to SO3: O2 to SO2:
3) PCl3 + Cl2 → PCl5
PCl3 to Cl2: PCl3 to PCl5:
4) 4 NH3 + 3 O2 → 2 N2 + 6 H2O
NH3 to N2: H2O to O2:
5) Fe2O3 + 3 CO → 2 Fe + 3 CO2
CO to CO2: Fe to CO:
Mole to Mole Practice Problems
Here's the equation to use for all three problems:
2 H2 + O2 → 2 H2O
1)How many moles of H2O are produced when 5.00 moles of oxygen are used?
Remember: Starting with five moles of oxygen and based on the balanced equation, for every 1 mole of oxygen used, two moles of water are produced.
______moles O2 x 2 mole H2O =
1 mole O2
2) If 3.00 moles of H2O are produced, how many moles of oxygen must be consumed?
3) Using the data from problem two, how many moles of hydrogen gas must be used?
------
More Mole-to-Mole Practice Problems
1. Base your answers to the questions below on the following equation:
2CO + O2 → 2CO2
a. Write all the mole ratios that exist between the three compounds:
b. How many moles of oxygen are needed to completely react with 5.43 moles of carbon monoxide?
c. How many moles of carbon dioxide are produced by the reaction of 1.12 moles of oxygen with an excess of carbon monoxide?
d. How many moles of carbon dioxide are produced by the reaction of 1.12 moles of carbon
monoxide with an excess of oxygen?
2. Base your answers to the questions below on the following equation:
2KBr + Cl2 → 2KCl + Br2
a. How many moles of bromine are produced when 0.056 moles of chlorine react with an
excess of potassium bromide?
b. How many moles of potassium chloride are produced when 0.056 moles of bromine are produced?
c. How many moles of chlorine are required to react completely with 2.45 moles of potassium
bromide?
How to do Gram-to-Gram Stoichiometry
(adapted from: https://chemfiesta.wordpress.com/2015/02/02/the-magic-of-stoichiometry/)
Key Point: Calculations for chemical reactions only occur on a mole to mole basis. You cannot compare reactions on a gram-to-gram basis. All grams must be converted to moles before the stoichiometry can be used.
How to do stoichiometry
Before we do anything, we’re going to make a modified version of the diagram we saw back when we were doing mole calculations:
Example: Using the BALACED equation:
2 H2 + O2→2 H2O
determine how many grams of water can be formed from 45.0 grams of oxygen and an excess of hydrogen gas. (Excess of hydrogen gas means you have more than enough hydrogen gas available for the reaction to run).
Step 1 - Start with your given information.
In this case, the problem tells you that you have 45.0 grams of oxygen, so write “45.0 grams of oxygen” on the top left.
Step 2: Since chemical reactions only occur on a mole-to-mole basis, we have to convert the given grams to moles using the molar mass of the compound or element.
Step 3: Add another section to the t, and write the units of the thing in the top left on the bottom right:
Step 4: Write the units of the thing you want to find in this step in the top right.
We’re converting from moles of oxygen to moles of water here, so write “moles of water” in the top right:
Step 5: This is the stoichiometry part - the molar ratio from the balanced equation.
Now, given that we have “moles” on both the top and the bottom, it doesn’t really make sense to put “1” in each spot as we usually do. Instead, realizing that the equation gives us a ratio of the number of moles of oxygen to number of moles of water (these are the coefficients in the equation), we’ll put these numbers in front of each number. This ratio is called the “molar ratio”, because it’s a ratio of moles.
Step 6: Since we want grams of water and we currently have moles, use the molar mass of water to convert to grams.
Step 7: Do the math: Multiply all the top numbers; multiply all the bottom numbers and divide.
Answer: 50.6 gram of water.
And that’s how you dostoichiometry!
Mass to Mass (Gram to Gram) Stoichiometry
For the following problems, calculate how much of the indicated product is made. SHOW ALL WORK! ONLY DIMENSIONAL ANALYSIS CAN BE USED!
1. If you start with 10.0 grams of lithium hydroxide, how many grams of lithium bromide will be produced?
LiOH + HBr → LiBr + H2O
2. How many grams of potassium chloride are produced if 25.0 g of potassium chlorate decompose?
2KClO3 → 2KCl + 3O2
3. How many grams of hydrogen are necessary to react completely with 50.0 grams of nitrogen?
N2 + 3H2 → 2NH3
How many grams of ammonia are produced in the reaction with 50.0 g of nitrogen in Problem 3?
And now! The moment you’ve been waiting for (even though you didn’t know it)…….
It’s time to put all your knowledge together!!!! You can do it!!!!
For the following:
- Write out the formulas for the chemical equation (write out the ions and criss-cross!)
- Balance the equation
- Solve the problem
4. Write the BALANCED chemical reaction between aqueous silver nitrate and barium chloride to form silver chloride and barium nitrate.
Type:______
a. How many grams of silver chloride are produced when 5.0 g of silver nitrate react?
b. How many grams of barium chloride is necessary to react with 7.5 grams of silver nitrate?
5. Write the BALANCED chemical equation for the reaction of gaseouos C2H4 and oxygen gas to form gaseous carbon dioxide and gaseous water.
Type:______
a. If you start with 45.0 grams of C2H4, how many grams of carbon dioxide will be produced?
b. If 68 grams of oxygen gas is available, how many grams of C2H4 is needed to react with that much oxygen?
______
6. Write the BALANCED chemical equation for the reaction between aqueous solutions of hydrochloric acid and Silver Nitrate to produce aqueous nitric acid and solid silver chloride.
Type:______
a. If you start with 20.0 grams of hydrochloric acid, how many grams of silver nitrate will you need?
b. If 45.0 grams of silver nitrate are reacted, how many grams of nitric acid will be produced?
______
7. Write the BALANCED chemical equation for the reaction between solid potassium and aqueous sodium fluoride to produce solid sodium metal and aqueous potassium fluoride.
Type:______
a. If you start with 5.5 grams of sodium fluoride, how many grams of potassium fluoride will be produced?
b. If 5.5 grams of sodium fluoride is reacted, how much sodium is produced?
______
8. Write the BALANCED chemical equation for the reaction between aqueous solutions of iron(III)chloride and sodium hydroxide to produce iron(III)hydroxide and sodium chloride.
Type:______
a. If 2.35 grams of iron(III)chloride reacts, how many grams of sodium hydroxide is needed?
b. If 1.67 grams of iron(III)hydroxide is produced, how many grams of sodium hydroxide would be needed?
9. Write the BALANCED chemical equation for the reaction between aluminum metal and copper(II)sulfate to produce aluminum sulfate and copper metal.
a. If 5.76 grams of copper(II)sulfate react, how many grams of aluminum is needed?
b. If 3.28 grams of aluminum sulfate are produced, how many grams of copper is also produced?
c. If 5.90 grams of copper is produced, how many grams of copper(II)sulfate was reacted?
13