We can further expand our understanding of stoichiometry by using the system of ratios we have created as conversions for questions that we are asked in chemistry. This section builds on the skills you have practiced starting in the unit conversions section and continued in the grams moles molecules and atoms lesson. These questions may seem strange to some students, but we use them in everyday activities. Baking and cooking are the best example I can think of. All baking recipes have specific ratios of ingredients. For example, a batch of cookies might require 2 eggs and 3 cups of sugar for 24 cookies. However, if you want to make 48 cookies (double the recipe. Then you need 2 * 2 = 4 eggs and 2 * 3 = 6 cups of sugar. We can display this information in a chemical equation and a stoichiometric conversion.
2 eggs + 3 cups of sugar —> 24 cookies
48 cookies | 2 eggs | = 4 eggs |
24 cookies |
(48 * 2)/24 = 4
48 cookies | 3 cups of sugar | = 6 cups of sugar |
24 cookies |
(48 * 3)/24 = 6
In the same way we use this technique for baking or cooking, we can also use it for chemistry. This is because baking and cooking are chemistry. In chemical stoichiometry, the unit we most often use is the MOLE. So, any coefficient in a chemical equation can be used to describe the mole-to-mole relationship. The unit of the mole is the currency of chemistry, the same way the dollar is the currency of the United States. This means that all transactions in chemistry eventually have to be converted into moles. Just like all transactions in the United States eventually have to be translated into dollars. This means that the mole conversions for stoichiometry are the most important conversions to learn. The picture below is a representation of what I am saying. It can be used as a map for guiding you through these conversions. As you work your way through chemistry problems use the map to guide you. Every step you take or every arrow that you pass through means you have to do 1 conversion. Right now it is a very basic map that should be very straightforward, but as you get deeper into chemistry this map will grow more and more complex. Remember your problem solving skills. Write down your information first.
VIDEO Stoichiometry Conversions Demonstrated Example 1: How many moles of Mg can be made from 5 moles of Na given the chemical equation below?
2 Na + MgBr_{2} ——> Mg + 2 NaBr
Step 1:
What information does the question supply us with?
Answer: 5 mol Na
Step 2:
What units does the question ask?
Answer: mol Mg
Step 3:
How many conversions must we do?
Answer: Look at the conversion map. We pass through 1 arrow when we go from moles of A —> moles of B. 1 arrows = 1 conversion
Step 4:
How do we set up the problem?
Answer:
5 mol Na | mol Mg | |
1 |
Step 5:
What is the first conversion?
Answer: mole to mole ratio
Step 6:
How do I put that in?
Answer: units first, set up the units that need to cancel out (in red)
5 mol Na | Mg = | mol Mg |
Na |
Step 7:
What is the next step?
Answer: Fill in the numbers and cross out units
5 mol Na | 1 Mg = | mol Mg |
2 Na |
Step 8:
How do I know I am done with conversions?
Answer: The only units left are the units that match the answer. In this case mol and Mg
5 mol Na | 1 Mg = | mol Mg |
2 Na |
Step 9: Simplify
5 mol | 1 Mg = | mol Mg |
2 |
Step 10:
How do I do the calculations?
Answer: (5 * 1) / 2 = 2.5
Step 11:
What is the complete answer?
COMPLETE ANSWER: 2.5 mol Mg
VIDEO Stoichiometry Conversions Demonstrated Example 2: If you have 13.4 mol of Ca then how many moles of Ag_{3}PO_{4} can you make?
6 Ag_{(s)} + Ca_{3}(PO_{4})_{2(s)} ——> 3 Ca_{(s)} + 2 Ag_{3}PO_{4(s)}
Step 1:
What information does the question supply us with?
Answer: 13.4 mol Ca
Step 2:
What units does the question ask?
Answer: mol Ag_{3}PO_{4}
Step 3:
How many conversions must we do?
Answer: Look at the conversion map. We pass through 1 arrow when we go from moles of A —> moles of B. 1 arrows = 1 conversion
Step 4:
How do we set up the problem?
Answer:
13.4 mol Ca | mol Ag_{3}PO_{4} | |
1 |
Step 5:
What is the first conversion?
Answer: mole to mole ratio
Step 6:
How do I put that in?
Answer: units first, set up the units that need to cancel out (in red)
13.4 mol Ca | Ag_{3}PO_{4} = | mol Ag_{3}PO_{4} |
Ca |
Step 7:
What is the next step?
Answer: Fill in the numbers and cross out units
13.4 mol Ca | 2 Ag_{3}PO_{4} = | mol Ag_{3}PO_{4} |
3 Ca |
Step 8: Simplify
13.4 mol | 2 Ag_{3}PO_{4} = | mol Ag_{3}PO_{4} |
3 |
Step 9:
How do I know I am done with conversions?
Answer: The only units left are the units that match the answer. In this case mol and Ag_{3}PO_{4}
13.4 mol | 2 Ag_{3}PO_{4} = | mol Ag_{3}PO_{4} |
3 |
Step 10:
How do I do the calculations?
Answer: (13.4 * 2) / 3 = 8.93
Step 11:
What is the complete answer?
COMPLETE ANSWER: 8.93 mol Ag_{3}PO_{4}
PRACTICE PROBLEMS: Calculate the moles you can obtain from the moles you are given. Use the conversion map table if you need it.
How many moles of BaCl_{2} can be made from 8 moles of HCl given the chemical equation below?
Ba(OH)_{2}_{(aq)} + 2 HCl_{(aq)} ——> 2 H_{2}O_{(l)} + BaCl_{2}_{(aq)}
Answer: 8 mol HCl
How many moles of (NH_{4})_{3}P can be made from 7 moles of NH_{4}F given the chemical equation below?
3 NiF_{2}_{(s)} + 2 (NH_{4})_{3}P_{(s)} ——> 6 NH_{4}F_{(aq)} + Ni_{3}P_{2}_{(s)}
Answer: 2.33 mol (NH_{4})_{3}P
If you have 10 mol of MnI_{3} then how many moles of Mn can you make?
2 MnI_{3}_{(s)} ——> 3 I_{2}_{(g)} + Mn_{(s)}
Answer: 5 mol Mn
If you have 4.2 mol of CO_{2} then how many moles of C_{4}H_{10 } can you make?
2 C_{4}H_{10(l) } + 13 O_{2(g) } ——> 8 CO_{2(g) } + 10 H_{2}O_{ (g)}
Answer: 1.05 mol C_{4}H_{10 }
How many moles of V_{3}P_{2} are required to completely react with 0.63 moles of Ag given the chemical equation below?
2 Ag_{3}P_{(s)} + 3 V_{(s)} ——> V_{3}P_{2(s)} + 6 Ag_{(s)}
Answer: 0.105 mol V_{3}P_{2}