## Chem – Combining Stoichiometry and Molar Mass Conversions

COMBINING STOICHIOMETRY AND MOLAR MASS CONVERSIONS (MOLES TO GRAMS CONVERSIONS):

Now that we are more comfortable with stoichiometry, we can combine it with our previous efforts of molar mass conversions in the section. This means we are going to further complicate our conversion map to include more destinations.

VIDEO Stoichiometry Conversions Demonstrated Example 3: If you have 4 mol of O2 then how many grams of H2 will you need to completely react? You will need a periodic table to help solve this problem.

2 H2(g) + O2(g) —-> 2 H2O(l)

Step 1:

What information does the question supply us with?

Step 2:

What units does the question ask?

Step 3:

How many conversions must we do?

Answer: Look at the conversion map. We pass through 2 arrows when we go from Moles of A —> Moles of B —> Grams of B. 2 arrows = 2 conversions

Step 4:

How do we set up the problem?

 4 mol O2 g H2 1

Step 5:

What is the first conversion?

Answer: mole to mole ratio (coefficient ratio)

Step 6:

How do I put that in?

Answer: units first, set up the units that need to cancel out (in red)

 4 mol O2 H2 g H2 O2

Step 7:

What is the next step?

Answer: Fill in the numbers and cross out units

 4 mol O2 2 H2 g H2 1 O2

Step 8:  Simplify

 4 mol 2 H2 g H2 1

Step 9:

What is the next conversion?

Answer: molar mass (grams to mole ratio) of H2 found on the periodic table

Step 10:

How do I set it up?

Answer: units first, set up the units that you need to cancel out (in red)

 4 mol 2 H2 g = g H2 1 mol

Step 11:

What is the next step?

Answer: Fill in numbers and cross out units

 4 mol 2 H2 2 g = g H2 1 1 mol

Step 12:  Simplify

 4 2 H2 2 g = g H2 1 1

Step 13:

How do I know I am done with conversions?

Answer: The only units left are the units that match the answer. In this case g and H2

 4 2 H2 2 g = g H2 1 1

Step 14:

How do I do the calculations?

Answer: (4 * 2 * 2) = 16

 4 2 H2 2 g = 16 g H2 1 1

Step 15:

VIDEO Stoichiometry Conversions Demonstrated Example 4: If you have 30g of CO2 then how many moles of C2H6 will be produced? You will need a periodic table to help solve this problem.

2 C2H6(l) + 7 O2(g) ——> 4 CO2(g) + 6 H2O(g)

Step 1:

What information does the question supply us with?

Step 2:

What units does the question ask?

Step 3:

How many conversions must we do?

Answer: Look at the conversion map. We pass through 2 arrows when we go from Grams of A —> Moles of A —> Moles of B. 2 arrows = 2 conversions

Step 4:

How do we set up the problem?

 30 g CO2 mol C2H6 1

Step 5:

What is the first conversion?

Answer: molar mass (grams to mole ratio) of CO2 found on the periodic table

Step 6:

How do I put that in?

Answer: units first, set up the units that need to cancel out (in red)

 30 g CO2 mol mol C2H6 g

Step 7:

What is the next step?

Answer: Fill in the numbers and cross out units

 30 g CO2 1 mol mol C2H6 44 g

Step 8:  Simplify

 30 CO2 1 mol mol C2H6 44

Step 9:

What is the next conversion?

Answer: mole to mole ratio (coefficient ratio)

Step 10:

How do I set it up?

Answer: units first, set up the units that you need to cancel out (in red)

 30 CO2 1 mol C2H6 mol C2H6 44 CO2

Step 11:

What is the next step?

Answer: Fill in numbers and cross out units

 30 CO2 1 mol 2 C2H6 mol C2H6 44 4 CO2

Step 12:  Simplify

 30 1 mol 2 C2H6 mol C2H6 44 4

Step 13:

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 C2H6

 30 1 mol 2 C2H6 = mol C2H6 44 4

Step 14:

How do I do the calculations?

Answer: (30 * 2) / (44 * 4) = 0.34

 30 1 mol 2 C2H6 = 0.34 mol C2H6 44 4

Step 15:

VIDEO Stoichiometry Conversions Demonstrated Example 5: If you have 12.9 g of Ca3(PO4)2 then how many grams of NaCl will you need to completely react? You will need a periodic table to help solve this problem.  You can also use the conversion map.

2 Na3PO4(aq) + 3 CaCl2(aq) ——> Ca3(PO4)2(s) + 6 NaCl(aq)

Step 1:

What information does the question supply us with?

Step 2:

What units does the question ask?

Step 3:

How many conversions must we do?

Answer: Look at the conversion map. We pass through 3 arrows when we go from Grams of A —> Moles of A —> Moles of B —> Grams of B. 3 arrows = 3 conversions

Step 4:

How do we set up the problem?

 12.9 g Ca3(PO4)2 g NaCl 1

Step 5:

What is the first conversion?

Answer: molar mass (grams to mole ratio) of Ca3(PO4)2 found on the periodic table

Step 6:

How do I put that in?

Answer: units first, set up the units that need to cancel out (in red)

 12.9 g Ca3(PO4)2 mol g NaCl g

Step 7:

What is the next step?

Answer: Fill in the numbers and cross out units

 12.9 g Ca3(PO4)2 1 mol g NaCl 310 g

Step 8:  Simplify

 12.9 Ca3(PO4)2 1 mol g NaCl 310

Step 9:

What is the next conversion?

Answer: mole to mole ratio (coefficient ratio)

Step 10:

How do I set it up?

Answer: units first, set up the units that you need to cancel out (in red)

 12.9 Ca3(PO4)2 1 mol NaCl g NaCl 310 Ca3(PO4)2

Step 11:

What is the next step?

Answer: Fill in the numbers and cross out units

 12.9 Ca3(PO4)2 1 mol 6 NaCl g NaCl 310 1 Ca3(PO4)2

Step 12:  Simplify

 12.9 1 mol 6 NaCl g NaCl 310 1

Step 13:

What is the next conversion?

Answer: molar mass (grams to mole ratio) of NaCl found on the periodic table

Step 14:

How do I put that in?

Answer: units first, set up the units that need to cancel out (in red)

 12.9 1 mol 6 NaCl g = g NaCl 310 1 mol

Step 15:

What is the next step?

Answer: Fill in the numbers and cross out units

 12.9 1 mol 6 NaCl 58 g = g NaCl 310 1 1 mol

Step 16:  Simplify

 12.9 1 6 NaCl 58 g = g NaCl 310 1 1

Step 17:

How do I know I am done with conversions?

Answer: The only units left are the units that match the answer. In this case g and NaCl

 12.9 1 6 NaCl 58 g = g NaCl 310 1 1

Step 18:

How do I do the calculations?

Answer: (12.9 * 6 * 58) / (310) = 14.5

 12.9 1 6 NaCl 58 g = 14.5 g NaCl 310 1 1

Step 19:

PRACTICE PROBLEMS: Calculate the moles or grams you can obtain from the moles or grams you are given.  You will need the periodic table.

If you have 3 moles of Mg then how many grams of O2 will you need?

2 Mg + O2 —> 2 MgO

If 25 g of Ca(OH)2 are used, how many moles of NaOH can you make?

Ca(OH)2 + Na2S —> CaS + 2 NaOH

How many grams of Al do you need if you have 8 g of Br2?

2 AlBr3 —> 2 Al + 3 Br2

If you have 0.6g of CO2, how many grams of O2 can you make?

2 C4H10 + 13 O2 —> 8 CO2 + 10 H2O  