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Stoichiometry, Limiting Reactants and Percent Yield.

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Presentation on theme: "Stoichiometry, Limiting Reactants and Percent Yield."— Presentation transcript:

1 Stoichiometry, Limiting Reactants and Percent Yield

2 How many grams of carbon dioxide will be produced when 11.6 g of butane (C 4 H 10 ) burns with oxygen? 2 C 4 H O 2  8 CO H 2 O 11.6 g ? g 2 mole 8 mole 11.6 g C 4 H 10 1 mole C 4 H 10 X g C 4 H 10 8 mole CO 2 X mole C 4 H g CO 2 X = 1 mole CO g CO 2

3 Excess/Limiting Reactants The limiting reactant is the reactant present in the smallest stoichiometric amount –In other words, it’s the reactant you’ll run out of first! The excess reactant will be left over after the reaction.

4 Limiting Reactants Example a) 11.6 grams of magnesium react with 54.6 grams of nitric acid. Hydrogen and magnesium nitrate are produced. How many grams of hydrogen are produced? 11.6 g 54.6 g x g Mg + 2 HNO > H 2 + Mg(NO 3 ) 2

5 Method 1 Here are the 2 possible calculations (you really only need to do one of these): 1 mol Mg 2 mol HNO g HNO g Mg X X X = 60.1 g HNO g Mg 1 mole Mg 1 mole HNO 3 We can’t use all the Mg – there’s not enough HNO 3 to react OR 1 mole HNO 3 1 mole Mg 24.3 g Mg 54.6 g HNO 3 X X X = 10.5 g Mg 63.0 g HNO 3 mole HNO 3 mole Mg If we use all the HNO 3 we’ll still have Mg left over

6 Method 1 cont. ALWAYS USE THE LIMITING REACTANT TO FIND THE REQUIRED! 1 mole HNO 3 1 mole H g H g HNO 3 X X X =.875 g H g HNO 3 2 mole HNO 3 1 mole H 2

7 Method 2 AND The smaller answer is correct – 0.875g of H 2 is formed. HNO 3 is the limiting reactant and Mg is the excess reactant We know this because we used HNO 3 to find the correct answer

8 Part b… b.How much magnesium is left over after the reaction? (you may have already done the first step if you happened to do this calculation to find the limiting reactant in method 1) 1 mole HNO 3 1 mole Mg 24.3 g Mg 54.6 g HNO 3 X X X = 10.5 g Mg 63.0 g HNO 3 mole HNO 3 mole Mg 11.6g Mg – 10.5 g Mg = 1.1 g Mg is left over!

9 Theoretical Yield The theoretical yield is the amount of product that can be made –This is what we calculate The actual yield is the amount one actually produces and measures –Either measured in the lab or given in the problem

10 Percent Yield A comparison of the amount actually obtained to the amount it was possible to make Actual Yield Theoretical Yield Percent Yield =x 100%


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