Answer:
The limiting reagent is HCl.
18.42 grams of Mg, the excess reagent, will not react.
Explanation:
The limiting reagent is one that is consumed first in its entirety, determining the amount of product in the reaction. When the limiting reagent is finished, the chemical reaction will stop.
To know the limiting reagent, you must first know the amount of mass that reacts by stoichiometry of the reaction (that is, the relationship between the amount of reagents and products in a chemical reaction). For that, you must know the molar mass (mass of one mole) of each reagent present in the reaction. Then you must know the mass of each element that makes up the compound. This is obtained in the periodic table:
- Mg: 24 g/mol
- H: 1 g/mol
- Cl: 35.45 g/mol
Then:
- Mg(s): 24 g/mol
- HCl(aq): 36.45 g/mol
By stoichiometry you can see that 1 mol of Mg (s) and 2 moles of HCl (aq) react. So the mass that reacts by stoichiometry is:
- Mg(s): 24 g/mol
- HCl(aq): 2*36.45 g/mol=72.9 g/mol
The rule of three or is a way of solving problems of proportionality between three known values and an unknown value, establishing a relationship of proportionality between all of them. That is, what is intended with it is to find the fourth term of a proportion knowing the other three. Remember that proportionality is a constant relationship or ratio between different magnitudes.
If the relationship between the magnitudes is direct, that is, when one magnitude increases, so does the other (or when one magnitude decreases, so does the other) , the direct rule of three must be applied. To solve a direct rule of three, the following formula must be followed:
a ⇒ b
c ⇒ x
[tex]x=\frac{c*b}{a}[/tex]
The rule of three can be used to calculate the limiting reagent as follows: if by stoichiometry 72.9 grams of HCl react with 24 grams of Mg, then if 20 grams of HCl react how many grams of Mg are required?
[tex]massofMg=\frac{20*24}{72.9}[/tex]
mass of Mg≅6.58 grams
But you have 25 grams of Mg available. That is, you have more mass available than are necessary to react with 20 grams of HCl. This means that Mg is the excess reagent. Therefore the limiting reagent is HCl.
But you have 25 grams of Mg available and only 6.58 grams are necessary to react with 20 grams of HCl. Then with a simple subtraction you can calculate the mass that does not react:
Mass remaining = 25 grams - 6.58 grams = 18.42 grams
Mass remaining = 18.42 grams
18.42 grams of Mg, the excess reagent, will not react.
