How many grams of mgcl2 will be produced from 12g of mg(oh)2 and 42g of hcl

Answer 1

In order to solve this problem you must know how to balance a chemical equations and how to calculate molar quantities.

A chemical equation is written with reactants on the left side and the products on the right. In this problem we can identify the reactants: Mg(OH)₂ and HCl. One of the products is given, but we must determine if another product is created. According to the Law of the Conservation of Mass we know that we cannot disregard the "leftover" oxygen and hydrogen. A common compound containing hydrogen and oxygen would be H₂O.

In a chemical equation the reactants are written from left to right on the left side and the products are written after an arrow on the right side.

Mg(OH)₂ + HCl → Mg(Cl)₂ + H₂O

Now we must "count" the individual atoms that are represented on each side of the equation to make sure they are equal. It is handy to set up a table to keep track as you are balancing.

R P R stands for the reactants and P stands for the products.

Mg 1 1 Notice that O atoms and Cl atoms have unequal numbers on either side of the reaction

Cl 1 2

O 2 1

H 2 2

To balance the equation we can only change the number of molecules, not the formula of any compound...so no switching of subscripts allowed. You can see that we will need more than one molecule of HCl in the reactants, so start by adding a coefficient of 2 to that molecule.

Now the equation would look like this:

Mg(OH)₂ + 2 HCl → Mg(Cl)₂ + H₂O

Now check the table again:

R P

Mg 1 1

Cl 2 2

O 2 1

H 4 2

We are getting closer but still have an unequal number of atoms of O and H. If we double the number of H₂O molecules we get:

Mg(OH)₂ + 2 HCl → Mg(Cl)₂ + 2 H₂O

Now recheck the table:

R P

Mg 1 1

Cl 2 2

O 2 2

H 4 4

Success! This is not an optional step...you can't go from grams to grams. You must go through what I like to call the "mole tunnel". Since reactions occur at the at the atomic level we must understand that our equation would read as :

1 molecule of Mg(OH)₂ combines with 2 molecules of HCL to produce 1 molecule of MgCl₂ and 2 molecules of H₂O.

Molecules are incredibly tiny, so chemists refer to "moles"* of substances so that they can be measured. In order to find how many grams of any substance are in a mole, you can use the periodic table to calculate the atomic mass** of each molecule.

Mg(OH)₂ has a molar mass of 58.31 ∴ 58.31g Mg(OH)₂ = 1 mole of Mg(OH)₂

HCl has a molar mass of 36.45 ∴ 36.45 g HCl = 1 mole of HCl

MgCl₂ has a molar mass of 95.21 ∴ 95.21 g MgCl₂ = 1 mole of MgCl₂

H₂O has a molar mass of 18.02 ∴ 18.02 g H₂O = 1 mole of H₂O

Next we must consider how many moles of each of the reactants are used in the reaction.

12 g of Mg(OH)₂ is only a portion of a mole. The relationshipship is proportional. I will show one example, and hopefully you can follow the steps to calculate the other reactant.

Set up a proportion with grams on the left side and moles on the right:

12g Mg(OH)₂/ 58.31g Mg(OH)₂ = "x" moles Mg(OH)₂ / 1 mole Mg(OH)₂

∴ 12g Mg(OH)₂ = 0.21 mol of Mg(OH)₂

After solving the proportion for the other quantity (42g HCl) you should find that

42 g HCl = 1.15 mol of HCl

( I nickname the above step as traveling through the "mole tunnel")

Since we have more moles of HCl we must consider only the limiting reactant which is Mg(OH)₂.

Returning to our "translation" of the chemical equation we know that since one molecule of Mg(OH)₂ will produce one molecule of 1 molecule of MgCl₂

likewise .21 mol Mg(OH)₂ will produce .21 mol MgCl₂

Now we know how many moles of MgCl₂ are produced, but question asks how many grams are produced. Time to go back through the "mole tunnel".

.21 mol MgCl₂/ 1 mole MgCl₂ = "x" grams MgCl₂/ 95.21 g MgCl₂

Solving this proportion we find that 0.21 mol MgCl₂ = 19.99 g of MgCl₂

I hope this helps!