A reaction between 6.5 g of zinc and 50 cm3 of 1.0 moldm-3 hydrochloric acid has produced zinc chloride solution and hydrogen gas. Calculate the maximum mass of zinc chloride formed.
[RAM: Zn, 65; Cl, 35.5]
Answer:
I gave this question to my students few weeks ago in a short quiz. Unfortunately for such a simple question, none of them got the answer right despite of how self-assured they were with it. Apparently they did a mistake by taking the number of moles of zinc as a basis to determine the number of moles of zinc chloride and its mass, instead of using the number of moles of hydrochloric acid to do so.
Here is why. Below is the chemical equation of the reaction.
Zn ✛ 2HCl → ZnCl2 ✛ H2
In the case of number of moles of zinc which is 0.1 mol (6.5 g / 65 gmol-1 ) used as a basis, it means based on chemical equation, 0.1 mol of zinc reacts with 0.2 mol of hydrochloric acid to produce 0.1 mol of zinc chloride. That is just impossible, because given the volume and the molarity of hydochloric acid, the maximum number of moles of hydrochloric acid involved in the reaction is only 0.05 mol (0.05 dm3 x 1.0 moldm-3).
Sometimes tricky question like this was designed to test students' understanding on chemical reaction.
Okay now back to the correct answer.
Step 1: The number of moles of hydrochloric acid
= Molarity x Volume of HCl
= 1.0 moldm-3 x 0.05 dm3
= 0.05 mol
Step 2: Find the number of moles of zinc chloride
From chemical equation
2 mol of HCl produces 1 mol of ZnCl2
Therefore, 0.05 mol of HCl produces 0.025 mol of ZnCl2
Step 3: Find the mass of zinc chloride formed
= Number of moles x RMM of zinc chloride
= 0.025 mol x [65 + 35.5x2] gmol-1
= 0.025 mol x 136 gmol-1
= 3.4 g of zinc chloride
Once in a while, I like to throw tricky question in the expectation that my students would make a mistake in their answer. so that they could learn a hard lesson from it. In my experience, it's a very effective teaching method.
In the case of number of moles of zinc which is 0.1 mol (6.5 g / 65 gmol-1 ) used as a basis, it means based on chemical equation, 0.1 mol of zinc reacts with 0.2 mol of hydrochloric acid to produce 0.1 mol of zinc chloride. That is just impossible, because given the volume and the molarity of hydochloric acid, the maximum number of moles of hydrochloric acid involved in the reaction is only 0.05 mol (0.05 dm3 x 1.0 moldm-3).
Sometimes tricky question like this was designed to test students' understanding on chemical reaction.
Okay now back to the correct answer.
Step 1: The number of moles of hydrochloric acid
= Molarity x Volume of HCl
= 1.0 moldm-3 x 0.05 dm3
= 0.05 mol
Step 2: Find the number of moles of zinc chloride
From chemical equation
2 mol of HCl produces 1 mol of ZnCl2
Therefore, 0.05 mol of HCl produces 0.025 mol of ZnCl2
Step 3: Find the mass of zinc chloride formed
= Number of moles x RMM of zinc chloride
= 0.025 mol x [65 + 35.5x2] gmol-1
= 0.025 mol x 136 gmol-1
= 3.4 g of zinc chloride
Once in a while, I like to throw tricky question in the expectation that my students would make a mistake in their answer. so that they could learn a hard lesson from it. In my experience, it's a very effective teaching method.
