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Lowell High School Chemistry

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Practice Problem #4:

Given the following reaction, answer Parts A and B.

NaClO3 (s) –>  NaCl (s) + O2 (g)                  [unbalanced]

Part A: 12.00 moles of NaClO3 will produce how many grams of O2?

Part B: How many grams of NaCl are produced when 80.0 grams of O2 are produced?

Answer to Practice Problem #4 (Part A):

2NaClO3 (s) –>  2NaCl (s) + 3O2 (g)                        [balanced]

2. Convert from grams to moles using molar mass.

12.00 moles of NaClO3 is already given so you don’t need to convert to moles.

3. Determine the limiting reagent [if necessary] (Use mole ratios to figure out.)

NaClO3 is the only reactant and therefore the limiting reagent in this reaction.

4. Use ratios to find the moles of the reactant or product you need to find.

What you are trying to find is grams of O2, so you need to find the moles of O2 first. The mole ratio comparing O2 to NaClO3 (as given in your balanced equation) is 3 mol O2 : 2 mol NaClO3.

12.00 mol NaClO3 X  ( 3 mol O2   / 2 mol NaClO3 )  = 18.00 mol O2 produced

5. Convert from moles back to grams of the new substance using that substance’s molar mass.

To get grams from moles of O2:          [molar mass = 32 g/mol]

18.00 mol O2    X  ( 32 g  /  1 mol ) = 576.00 g O2 produced

Answer to Problem #4 (Part A): 576.00 g O2 produced

Answer to Practice Problem #4 (Part B):

2NaClO3 (s) –>  2NaCl (s) + 3O2 (g)                        [balanced]

2. Convert from grams to moles using molar mass.

To get grams from moles of O2:          [molar mass = 32 g/mol]

80.00 g O2   X  ( 1 mol / 32 g ) = 2.50 mol O2 produced

3. Determine the limiting reagent [if necessary] (Use mole ratios to figure out.)

Not necessary.

4. Use ratios to find the moles of the reactant or product you need to find.

What you are trying to find is grams of NaCl produced, so you need to find the moles of NaCl first. The mole ratio comparing NaCl to O2 (as given in your balanced equation) is 2 mol NaCl : 3 mol O2.

2.50 mol O2 X  ( 2 mol NaCl   / 3 mol O2 )  = 1.666667 mol NaCl produced

5. Convert from moles back to grams of the new substance using that substance’s molar mass.

To get grams from moles of NaCl:      [molar mass = 58.45 g/mol]

1.666667 mol NaCl  X  ( 58.45 g  /  1 mol ) = 97.417 g NaCl produced

Answer to Problem #4 (Part B): 97.417 g NaCl produced

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