Students in a chemistry class added 5 g of Zinc (Zn) to 50 g of hydrochloric acid (HCl). A chemical reaction occurred that produced zinc chloride (ZnCl2) and 6 g of hydrogen gas (H2).

Question

Students in a chemistry class added 5 g of Zinc (Zn) to 50 g of hydrochloric acid (HCl). A chemical reaction occurred that produced zinc chloride (ZnCl2) and 6 g of hydrogen gas (H2). 

Zn+ HCl → ZnCl2 + H2

What is the mass of Zinc Chloride (ZnCl₂)? 

1. A.49 g
2. B.55 g
3. C.61 g
4. D.Not enough information to determine the mass of ZnCl₂

Answer 1

Hello, Kimberly,

I note that Paul J. has provided good information on this problem already. I would like to add that I'm unable to understand how 6 grams of H₂ could have been produced from only 5 grams of Zn. I agree with Pauk that the equation is not balanced, but we only need to add a coefficient of 2 in front of the HCl to bring it in line. The problem I'm having is that 5 grams of Zn is only 0.0765 moles of Zn. The equation says we'll get 1 mole of H₂ for every mole of Zn, a 1 to 1 molar ratio. Therefore I'd expect to see 0.0765 moles of H₂ Given the molar mass of hydrogen is 2, that only amounts to 0.153 grams of H₂. Is the mass of Zn correct? Perhaps the amount of hydrogen is incorrect?

Assuming the numbers are correct, then the balanced equation says we will get 1 mole of ZnCl₂ for every 1 mole of Zn, which is 0.0765 moles. That would amount to 10.4 grams of ZnCl₂, which is not an answer option. If we assume the 6 g of hydrogen is correct, this is 3 moles of H₂. The molar ratio of ZnCl₂ to H₂ is 1 to 1, so we'd expect 3 moles of ZnCl₂. That converts to 408 grams of zinc chloride, also not an option.

Sorry, but I don't understand how the numbers given in the problem could result in any of the posted answer options. Perhaps I'm overlooking something, but pwehaps you can track my logic and find a resolution.

Bob

Answer 2

Hello, I can help you set up this problem, but you are going to have to solve this on your own.

So this problem is a stoichiometry problem involving limiting reagents. There are multiple steps to solving this problem:

Step 1: Make sure the provided reaction equation is balanced.

- The reaction you provided is not balanced, because on the reactants side you have: 1 zinc atom, 1 hydrogen atom, and 1 chlorine atom. On the products side: you have 1 zinc atom, 2 chlorine atoms, and 2 hydrogen atoms. The zinc atoms are balanced, but you will have to balance the hydrogen and chlorine atoms so you have the same number of each atom on both sides of the equation.

Step 2: The easiest way to go about solving this problem is to convert the provided masses of reagents and products to moles before doing any calculations. In order to do this, you will need to know how to write conversion factors and know the molar masses of the reagents and products.

The easiest way to write conversion factors is to use the following format:

Given unit = (Desired Unit / Given Unit) = Desired Unit

This format ensures that the given units cancel and leave you with the unit you are trying to solve for.

Remember that molar masses are expressed in grams/mol. You already know how much of each reactant and product you have in grams (your given unit), you just have to convert them to moles (your desired unit).

Step 3: Determine your limiting reagent

1. Please note that you cannot complete this step without balancing the equation as mentioned in step 1, and converting the masses into moles as mentioned in step 2 (you need to know the stoichiometric coefficients which can only be determined by balancing the equation)
2. The limiting reagent is the reactant that is completely consumed by the time the reaction has completed, which ultimately determines how much product is formed.
3. Once you have your stoichiometric coefficients and molar equivalents of reactants and products, you must calculate the number of moles of one product that would be required to consume all of the moles of the other product. This will help you to determine which product is the limiting reagent.
4. I will provide an example of this using the combustion of glucose:
5. Reaction: C6H12O6+6O2→6CO2+6H2O (please note this is a balanced eqn.)
6. Now lets say we have 1.25 moles of oxygen and 0.14 moles of glucose (C6H12O6)
7. In this reaction, glucose reacts in a 1:6 ratio to produce 6 moles of carbon dioxide and 6 moles of water vapor.
8. If we assume that all 1.25 moles of oxygen are consumed by this reaction, using the stoichiometric coefficients, we can determine the amount of glucose needed to consume all 1.25 moles of oxygen:
9. 1.25 moles oxygen (1 mole glucose / 6 moles oxygen) = 0.21 moles of glucose
10. So 0.21 moles of glucose are needed to consume all 1.25 moles of oxygen.
11. Now if we assume that all 0.14 moles of glucose are consumed:
12. 0.14 moles glucose (6 moles oxygen / 1 mole glucose) = 0.83 moles of oxygen.
13. In this reaction, we only have 0.14 moles of glucose. So there is no way that all of the oxygen could have been consumed because it would require 0.21 moles of glucose.
14. This means that glucose is our limiting reagent because we don't have enough glucose to consume all of the oxygen, but we do have enough oxygen to consume all of the glucose which would leave us with excess oxygen.

Step 4: Determine the theoretical yield of product using the stoichiometric coefficients and the moles of limiting reagent.

1. Say we want to determine the amount of carbon dioxide produced.
2. Glucose will react to produce carbon dioxide in a 1:6 ratio (1 mole of glucose produces 6 moles of CO2).
3. You would write your stoichiometric calculation as follows:
4. 0.14 moles glucose (6 moles CO2 / 1 mole glucose) = 0.83 moles of carbon dioxide produced.

Now in your case, it is asking you to calculate the number of grams of product produced by your reaction. You will have to include an extra conversion factor using the molar mass of zinc chloride to convert from moles to grams.

I hope this helps. Please comment if you need anything clarified. Feedback and questions are always welcome and appreciated :).