5.05 g of solid Mg(OH)2 is added to 34.2 mL of 0.498 M H2SO4 solution. What will the concentration of Mg2+ be when all of acid has been neutralized? How many grams of Mg(OH)2 will not have dissolved?

Question

Very confused on this please help.

Answer 1

Hello,

Questions like these want you to think through all of the different steps and work through them one at a time. A good place to start is to write out the chemical equation and try to balance it.

I know the two reactants: Mg(OH)2 and H2SO4

This is an Acid-Base Reaction. You should be able to see that the Hydrogen from the Sulfuric Acid will be donated to the Magnesium Hydroxide. This will make Water and a different Salt. It's okay if you don't see that right away but you should be on the lookout for these kinds of reactions.

Mg(OH)2 + H2SO4 -> H2O + MgSO4

This is the simplest equation that you can make with these reactants and products but you should always check to see if the equation is balanced. In this case it is NOT balanced so you will have to add coefficients to the molecules until both sides have exactly the same number of each type of Atom.

(Hint: You only need to change 1 coefficient and the equation will be fully balanced)

When the question asks for the concentration of Magnesium Ion it is a little confusing. Both Magnesium Hydroxide and Magnesium Sulfate are Ionic Compounds. But what you should know from the Solubility Rules is that Hydroxide Ions are usually not very good at dissolving. This means that the question is asking about the dissolved Magnesium Ion from the Magnesium Sulfate (The Product).

Another problem you have to look at is whether or not you have the right amount of acid. If there isn't enough Acid some of the Magnesium Hydroxide you started with won't be used up and it will still be stuck in a molecule with the Hydroxides.

To determine the amount of product created you need to convert everything to moles and then see which compound is the limiting reagent. This is a long step but you can break it down even further.

1) Find the Molar Masses of the 2 different Reactants

2a) Convert 5.05 g of Mg(OH)2 to moles

2b) Convert 34.2 mL of H2SO4 to moles. Here the concentration of the acid is 0.498 Molarity. That means that the conversion rate will be 0.498 moles per Liter. Don't forget to convert milliliters to liters first.

3) Use the Balanced Equation to determine which compound is the limiting reagent. If it is the Mg(OH)2 then all of the Magnesium will be Ionic. If it is the H2SO4 then there will be some left over.

4) Once you have the moles of Product you can convert it to a Molarity. Remember, the only water we have is the 34.2 mL of Acid so the concentration will be moles of Magnesium Ion in that amount of water.

5) If there were any moles of Magnesium Hydroxide that were not dissolved multiply that by the molar mass to get the grams of reactant that were not dissolved.

I understand that this is a complicated problem. Work on it one step at a time and don't forget to check your work. If you need to stop and go back over something you don't understand it's okay to review. Don't get discouraged and try your best. If you need any more help just ask. Good Luck.