How many grams of hydrogen gas will be produced from the reaction of zinc metal with 85 grams of hydrochloric acid

Question

Very confused and teacher is not here

Answer 1

First write an unbalanced chemical reaction equation - just get all the compounds in it and don't worry about how much of what you get.

You are told that the reactants are Zinc metal, that's Zn, and hydrochloric acid, that's HCl. And you are told that one product is hydrogen gas, that's H₂. So: Zn + HCl → H₂ but you'll notice we are missing Zn and Cl on the right side of the equation. We can assume that ZnCl is also a product making:

Zn + HCl → H₂ + ZnCl

Now, balance the individual molecules for ionic charge. Zn has a +2 ionic charge and Cl has a -1 ionic charge so there must be 2 chlorines to balance a zinc making the ZnCl into a ZnCl₂. Everything else is OK. So now we have:

Zn + HCl → H₂ + ZnCl₂

Now, balance the equation:

There are 2 H's on the right side and only 1 on the left so add a "2" coefficient in front of the HCl:

Zn + 2HCl → H₂ + ZnCl₂

No everything balances so this is the balanced chemical reaction equation.

You are given that there are 85g of HCl. But the chemical reaction equation is in moles, not grams. So you must convert the 85g to moles. To do that, you need the molar mass of HCl.

From the periodic table: H = 1.008 g/mol and Cl is 35.453 g/mol so HCl is 1.008 + 35.453 = 36.461 g/mol

Now divide 85 grams by 36.461 g/mol to get 2.33126 moles of HCl.

Using the balanced chemical reaction equation you see that for every 2 moles of HCL, there is 1 mole of H₂ gas produced. So divide the 2.33126 moles of HCl by 2 to get 1.16563 moles of H₂ gas.

Now you must convert the moles of H₂ gas into grams of H₂ gas. The molar mass of H₂ is 1.008 x 2 = 2.016 g/mol. Now multiply the 1.16563 moles of H₂ gas by the molar mass of 2.016 g/mol to get 2.3 grams of H₂ gas

Answer 2

Hi, Carlie!

The first step in any problem of this type is to determine the equation for your reaction. In this case, we know that zinc (Zn) and hydrochloric acid (the combination of hydrogen and chlorine, HCl) are the reactants. We also know that hydrogen gas is a product. Hydrogen (and most gasses) likes to exist in groups of two, so instead of having H as the product, it will be H₂. Our final product will be a combination of Zn and Cl. Zn can have variable charge, but is usually +2. Cl has a charge of -1. To make their charges equal 0, we'll need 2 Cl, so ZnCl₂.

Zn + HCl --> H₂ + ZnCl₂

We need to balance this equation. There are 2 Hs and 2 Cls on the products side, but only one of each on the reactants. We can fix this by putting a 2 in front of HCl.

Zn + 2HCl --> H₂ + ZnCl₂

This balanced equation is important to finding the bridge from one substance to another because we use the coefficients in front of each substance as a comparison point.

Now that we have the equation, we can begin the analysis portion. We know that we're starting with grams of HCl and trying to move to grams of H₂. I have to find the pathway to get there. I can use g of HCl to find moles of HCl, moles of HCl to find moles of H₂ (by using the coefficients from the chemical equation), and moles of H₂ to find g of H₂.

How do I use g of a substance to find moles of that substance? By comparing the g provided to the g of all the elements within that substance combined, using the atomic masses from the periodic table. Doing this, I find that HCl has a mass of 36.45 g / mol (35.45 g from Cl and 1 g from H) and that H₂ has a mass of 2 g / mol (1 g from each H atom).

Now I just set up my equation so that the unit I want to move toward is in the numerator of each fraction and the unit I want to move away from is in the denominator.

85 g HCl ( 1 mol HCl / 36.45 g HCl) (1 mol H₂ / 2 mol HCl) (2 g H₂ / 1 mol H₂)

Cancelling out all of the units leaves g of H₂ as your final unit, which is exactly what you want. Finally, multiplying out by the fractions gives an answer of 2.33 g of H₂.

I hope this helped!