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: <u>Calculate Moles of CaCO₃:</u>
Moles = Mass / M.Mass
Moles = 20.0 g / 100.08 g/mol
Moles = 0.199 moles of CaCO₃
Step 2: <u>Calculate theoretical amount of CaO produced;</u>
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: <u>Calculate %age Yield as;</u>
%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: <u>Calculate Moles of Na:</u>
Moles = Mass / M.Mass
Moles = 2.3 g / 23 g/mol
Moles = 0.10 moles of Na
Step 2: <u>Calculate theoretical amount of NaCl produced;</u>
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: <u>Calculate Actual Yield as;</u>
%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: <u>Calculate Moles of Mg:</u>
Moles = Mass / M.Mass
Moles = 100 g / 24.30 g/mol
Moles = 4.11 moles of Mg
Step 2: <u>Calculate theoretical amount of MgCl₂ produced;</u>
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: <u>Calculate %age Yield as;</u>
%age Yield = Actual Yield / Theoretical Yield × 100
%age Yield = 310.0 g / 391.73 g × 100
%age Yield = 79.13 %
