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Answer is: the average atomic mass 217.606 amu.
Ar₁= 203.973 amu; the average atomic mass of isotope.
Ar₂ = 205.9745 amu.
Ar₃ = 206.9745 amu.
Ar₄ = 207.9766 amu.
ω₁ = 1.40% = 0.014; mass percentage of isotope.
ω₂ = 24.10% = 0.241.
ω₃ = 22.10% = 0.221.
ω₄ = 57.40% = 0.574.
Ar = Ar₁ · ω₁+ Ar₂ · ω₂ + Ar₃ · ω₃ + Ar₄ · ω₄.
Ar = 203.973 amu · 0.014 + 205.9745 amu · 0.241 + 206.9745 amu · 0.221 + 207.9766 amu · 0.574.
Ar = 2.855 amu + 49.632 amu + 45.741 amu + 119.378 amu.
Ar = 217.606 amu.
But abundance of isotopes is greater than 100%.
It should be lead, with the fourth isotope weighs 207.9766 amu and an abundance of 52.40.
The yield of lithium chloride is 1.92 grams.
Option D.
<h3><u>Explanation:</u></h3>
In this reaction, we can see that 1 mole of lithium hydroxide reacts with 1 mole of potassium chloride to produce 1 mole of lithium chloride and 1 mole of potassium hydroxide.
Molecular weight of lithium hydroxide is 24.
Molecular weight of lithium chloride is 42.5.
So 24 grams of lithium hydroxide produces 42.5 grams of lithium chloride.
So, 20 grams of lithium hydroxide produces 
grams =11. 29 grams of lithium chloride.
But this is when the yield is 100%.
But yield is 17%.
So the yield is 1.92 grams of lithium chloride.
The answer is C. They lower the activation energy of an elementary step of a reaction
This makes the reaction rate to increase since less energy is required to make a reaction occur.
