if 20.2g of zn react with and equal amount of magnesium chloride to form magnesium and zinc chloride[zn+mgcl2>
mg+zncl2]
a.what is the limiting reactant?
b. how much magnesium will be produced?
Here is the way to solve most of these stoichiometry problems.
Step 1. Write the balanced equation.
Zn + MgCl2 ==> Mg + ZnCl2
Step 2. Convert what you have into mols.
2a. mols Zn = 20.2/65.4 = 0.309
2b. mols MgCl2 = 20.2/95.2 = 0.212
Step 3. Using the coefficients in the balanced equation, convert each of the mols from what you have (in this case mols Zn and mols MgCl2) to mols of what you want (in this case Mg is the product).
3a. First from 2a (mols Zn).
mols Mg = mols Zn x (1 mol Mg/1 mol Zn) = 0.309 x 1/1 = 0.309 mols Mg product.
3b. from 2b (mols MgCl2).
mols Mg = mols MgCl2 x (1 mol Mg/1 mol MgCl2) = 0.212 x 1/1 = 0.212.
3c. Now you make a decision. Both answers can't be correct; i.e., 3a says 0.309 mol Mg will be produced while 3b says 0.212 mols Mg will be produced. The answer ALWAYS is the smaller one AND MgCl2, then, is the limiting reagent. (That answers part a of the problem.)
Step 4. The problem doesn't state if you want mols, grams, or what. If you want mols, then 0.212 mol Mg will be produced. If you want grams, then
mols Mg x atomic mass Mg = grams Mg.
If you haven't seen this method before, it might be good to learn it. It will work literally hundreds of problems for you. If you don't have a limiting reagent problem, and many are not, simply omit steps 2b, 3b, and 3c.
You need to go through and correct any numbers for atomic masses of Zn, Mg, and molar mass MgCl2 for I estimated those values. They are close but may not be the best. Look them up on the periodic table.
Finally, I want you to know that this reaction will not take place but the reverse one will. That is, Mg will displace Zn from ZnCl2 in this reaction
Mg + ZnCl2 ==> Zn + MgCl2 but the reaction will not take place as it is writtenin the problem. Technically, then, the answer to the problem is that NO Mg will be produced; however, I don't think that is the intent of the problem. The steps I showed are the actual steps you will need for a valid reaction and they work equally well for reactions that will not occur.
To solve this problem, we need to use stoichiometry to determine the limiting reactant and the amount of magnesium that will be produced.
Step 1: Write the balanced equation.
The balanced equation for the reaction is:
Zn + MgCl2 → Mg + ZnCl2
Step 2: Convert the given amount of Zn into moles.
Using the molar mass of Zn, we can calculate the moles of Zn.
molar mass of Zn = 65.4 g/mol
moles of Zn = mass of Zn / molar mass of Zn = 20.2 g / 65.4 g/mol = 0.309 mol
Step 3: Convert the given amount of MgCl2 into moles.
Using the molar mass of MgCl2, we can calculate the moles of MgCl2.
molar mass of MgCl2 = 95.2 g/mol
moles of MgCl2 = mass of MgCl2 / molar mass of MgCl2 = 20.2 g / 95.2 g/mol = 0.212 mol
Step 4: Determine the limiting reactant.
To determine the limiting reactant, compare the stoichiometric ratio of the reactants in the balanced equation. The reactant that produces fewer moles of the desired product is the limiting reactant. In this case, we compare the moles of Zn and MgCl2.
From Step 3, we have 0.309 moles of Zn.
From Step 4, we have 0.212 moles of MgCl2.
Since the moles of MgCl2 are lower, MgCl2 is the limiting reactant.
Step 5: Calculate the moles of magnesium produced.
The limiting reactant determines the amount of product that can be formed. Since MgCl2 is the limiting reactant, we will use the stoichiometry from the balanced equation to calculate the amount of magnesium produced.
From the balanced equation, the stoichiometric ratio between MgCl2 and Mg is 1:1.
moles of Mg = moles of MgCl2 = 0.212 mol
If you need the amount in grams, you can use the molar mass of magnesium to convert moles to grams.
molar mass of Mg = 24.3 g/mol
mass of Mg = moles of Mg x molar mass of Mg = 0.212 mol x 24.3 g/mol = 5.144 g
Therefore, 5.144 grams of magnesium will be produced when 20.2 grams of zinc react with an equal amount of magnesium chloride.