Question

Percentage Yield

Answer 1

Q.No. 1:

            You have 20.0 g of CaCO₃. You decompose it by heat, and weigh the calcium oxide that  remains. You have 10.3 grams. What is the % yield?

Answer:

               %age Yield = 92.37 %

Solution:

               The balance chemical equation for given decomposition reaction is;

                                           CaCO₃ → CaO + CO₂

Step 1: Calculate Moles of CaCO₃:

                Moles  =  Mass / M.Mass

                Moles  =  20.0 g / 100.08 g/mol

                Moles  =  0.199 moles of CaCO₃

Step 2: Calculate theoretical amount of CaO produced;

According to equation,

              1 moles of CaCO₃ produced  =  1 mole of CaO

So,

           0.199 moles of CaCO₃ will produce  =  X moles of CaO

Solving for X,

                     X  =  0.199 mol × 1 mol / 1 mol

                     X  =  0.199 mol of CaO

Also,

         Mass  =  Moles × M.Mass

         Mass  =  0.199 mol × 56.07 g/mol

         Mass  =  11.15 g of CaO

Step 3: Calculate %age Yield as;

         %age Yield  =  Actual Yield / Theoretical Yield × 100

         %age Yield  =  10.3 g / 11.15 g × 100

         %age Yield = 92.37 %

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Q.No. 2:

            A student makes sodium chloride by buming 2.3 grams of sodium in chlorine  gas. If the yield is 90%, how much sodium chloride is made?

Answer:

               Actual Yield =  5.785 g of NaCl

Solution:

               The balance chemical equation for given decomposition reaction is;

                                           2 Na + Cl₂ → 2 NaCl

Step 1: Calculate Moles of Na:

                Moles  =  Mass / M.Mass

                Moles  =  2.3 g / 23 g/mol

                Moles  =  0.10 moles of Na

Step 2: Calculate theoretical amount of NaCl produced;

According to equation,

              2 moles of Na produced  =  2 moles of NaCl

So,

           0.10 moles of Na will produce  =  X moles of NaCl

Solving for X,

                     X  =  0.10 mol × 2 mol / 2 mol

                     X  =  0.10 mol of NaCl

Also,

         Mass  =  Moles × M.Mass

         Mass  =  0.10 mol × 58.44 g/mol

         Mass  =  5.844 g of NaCl

Step 3: Calculate Actual Yield as;

         %age Yield  =  Actual Yield / Theoretical Yield × 100

Or,

         Actual Yield  =  %age Yield × Theoretical Yield ÷ 100

         Actual Yield  =  99 × 5.844 g ÷ 100

         Actual Yield =  5.785 g of NaCl

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Q.No. 3:

            For the chemical reaction Mg(s) + 2 HCl (aq) → H₂ (g) + MgCl₂ (aq) calculate the  % yield if 100 grams of magnesium react with excess HCl to produce 310 grams of MgCl₂.

Answer:

               %age Yield = 79.13 %

Solution:

               The balance chemical equation for given decomposition reaction is;

                                           Mg + 2 HCl → MgCl₂ + H₂

Step 1: Calculate Moles of Mg:

                Moles  =  Mass / M.Mass

                Moles  =  100 g / 24.30 g/mol

                Moles  =  4.11 moles of Mg

Step 2: Calculate theoretical amount of MgCl₂ produced;

According to equation,

              1 mole of Mg produced  =  1 mole of MgCl₂

So,

           4.11 moles of Mg will produce  =  X moles of MgCl₂

Solving for X,

                     X  =  4.11 mol × 1 mol / 1 mol

                     X  =  4.11 mol of MgCl₂

Also,

         Mass  =  Moles × M.Mass

         Mass  =  4.11 mol × 95.21 g/mol

         Mass  =  391.73 g of MgCl₂

Step 3: Calculate %age Yield as;

         %age Yield  =  Actual Yield / Theoretical Yield × 100

         %age Yield  =  310.0 g / 391.73 g × 100

         %age Yield = 79.13 %