Chemistry Homework (very confusing and teacher was not here to teach it to us today)

Question

Hydrogen gas is produced when zinc reacts with hydrochloric acid. What is the limiting reactant if 8.89 g of zinc are used and 3.25 grams of HCL are used in the reactants. If the actual yield of this reaction is 85%, how many grams of zinc chloride are needed to produce this yield.

Answer 1

Zn + 2HCl —> ZnCl2 + H2

 

to determine the limiting reactant, we first find the moles Of each reactant by dividing grams present by its molar mass:

 

8.89 g Zn x (1 mole Zn/65 g Zn)= 0.137 moles Zn

 

3.25 g HCl x (1 mole HCl/36 g HCl) = 0.0903 moles HCl

 

we then convert moles of one to moles of the other using their ratio from the balanced equation. This tells us how much of the other reactant we would theoretically need to fully react with whats present:

 

0.137 moles zn x (2 mole HCl/1 mole zn) = 0.274 moles HCl

 

since we only have 0.0903 moles hcl (compared to the 0.274 moles we need), we will run out of hcl before all of the zn is fully reacted. Therefore hcl is limiting.

 

to solve the rest of the problem, we need to find the theoretical yield. This is done using the mole ratio again, this time for limiting reactant to product:

 

0.0903 moles hcl x (1 mole zncl2/ 2 mole hcl) = 0.04515 moles zncl2

 

then we convert that value to mass of zncl2 that can be formed by multiplying the theoretical moles by its molar mass:

 

0.04515 moles zncl2 x (136 g/1 mole zncl) = 6.14 g zncl2 

 

if the actual yield is 85%, the actual yield can be calculated by rearranging the equation for percent yield:

 

(actual/theoretica) x 100 = 85%

actual/6.14 = 0.85

actual = 5.23 g