Josh is measuring the enthalpy change associated with the reaction of Mg metal w
ID: 486555 • Letter: J
Question
Josh is measuring the enthalpy change associated with the reaction of Mg metal with aqueous hydrochloric acid. He adds, o.397 g of Mg ribbon to 95.25 mL of 1.00 M aqueous HCl solution. Determine first the limiting reagent, and answer the number of moles of limiting reagent that will react, assuming the reaction proceeds to completion Provide your response to three digits after the decimal.u will have 2 attempts. Question 5 0.5 pts Josh is measuring the enthalpy change associated with the reaction of Mg metal with aqueous hydrochloric acid. He adds o.397 g of Mg ribbon to 95.25 mL of 1.00 Maqueous HCI solution. Determine first the limiting reagent, and answer the number of moles of limiting reagent that will react, assuming the reaction proceeds to completion. Provide your response to three digits after the decimal. You may wish to consult a periodic table Do not use units (i.e. moles) in your answer. Previous Next
Explanation / Answer
Ans. Balanced Reaction: Mg(s) + 2HCl(aq) ----------> MgCl2(aq) + H2
Stoichiometry: 1 mol Mg reacts with 2 mol HCl to produce 1 mol each of MgCl2 and H2.
Theoretical stoichiometric ratio of reactants = mol of Mg : Mol of HCl = 1: 2
Now,
Moles of Mg in sample = Mass of Mg metal / Atomic mass of Mg
= 0.397 g / (24.305 g mol-1)
= 0.016334 mol
Moles of HCl in solution = Molarity x volume (in Liters) of HCl solution
= 1.00 M x 0.09525 L ; [1 L = 1000 mL]
= (1.00 mol/ L) x 0.09525 L ; [M = mol/ L]
= 0.09525 mol
So far, we have,
Moles of Mg available for reaction = 0.016334 mol
Moles of HCl available for reaction = 0.09525 mol
Strategy: We diving both the values by a factor “0.016334” to get the available molar ratio similar to that of theoretical molar ration. Be noted that diving or multiplying a ratio does not affect the ratio.
So,
Available molar ratio of reactant = Mol of Mg : Mol of HCl
= 0.016334: 0.09525 mol
= (0.016334/ 0.016334) : (0.09525: 0.016334)
= 1 : 5.83
We have converted the available molar ratio is a much better form, that is excellently suitbale for comparison.
Determining the Limiting reactant:
We have,
I. Theoretical stoichiometric ratio of reactants = mol of Mg : Mol of HCl = 1: 2
II. Available molar ratio of reactant = Mol of Mg : Mol of HCl = 1: 5.83
Compare 1 and II, the number of moles of HCl provided in the reaction mixture is greater than its theoretical requirement when moles of Mg is kept constant.
So,
HCl is the reactant in excess, and
Mg is the limiting reactant because its number of moles of lower that the actual value required to completely react with 5.83 mol HCl.
Moles of Mg available for reacting for completion = 0.016
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