Chemistry Question?

Question

I am working on my Chemistry Homework and need a little help. Can someone show me how to work out this problem.

What is the hydroxide ion concentration of a solution prepared by dissolving 0.731 g of Mg (OH) 2 in 227 mL of water?

Answer 1

Concentration is expressed as moles per liter. The problem states how many liters there are: 0.227 L (= 227 mL) So, we need to find out how many moles of hydroxide (OH) there are in 0.731 g of Mg(OH)2. The formula weight of Mg(OH)2 is 58.32 g/mol; so the number of moles is (0.731 g Mg(OH)2) / (58.32 g/mol) or 0.0125 moles. But wait! There's more to it in this case. When it is dissolved in water, Mg(OH)2 breaks into its component ions: one Mg ions and 2 hydroxide (OH) ions. 0.0125 moles * 2 OH/molecule = 0.025 moles OH, so there are really 0.0250 moles of hydroxide ions in solution. Now we know the number of moles of hydroxide, we go back to the molarity formula: 0.0250 moles OH / 0.227 L = 0.110 M hydroxide

Answer 2

If this is a high school question, you may accept the previous answer by armchairpoli.

If it is a college-level question, you need to find the disassociation constant for magnesium hydroxide, and factor that into your final answer.

A little on disassociation constant:
strong acids and bases disassociate almost 100%, e.g. 2 moles of HCl ----> 2 moles of H+ ions; 2 moles of H2SO4 ----> 4 moles of H+ ions; 2 moles of NaOH ----> 2 moles of OH - ions, etc.
Other acids and bases do not disassociate to this extent. To further complicate matters, if they have two of the same group, e.g. Mg(OH)2, the first OH - will disassociate to a certain extent, say 65 %, while the second OH - will disassociate to a less extent, say 48 %. The disassociation constants are not given in percentages; I just used that to explain the concept.

Answer 3

Here are the steps to work out your problem:

1. Convert mass of Mg(OH)2 to moles.

2. Notice that there are two moles of OH- for every mole of Mg(OH)2.

3. Calculate concentration of Hydroxide ion from c = n/V

I hope this helps. If you need more detail, let's know.

Answer 4

I'm happy to see you admit you need help with a homework problem and are not simply asking for an answer but rather asking for how to work it out. I one time tutored a guy in organic chemistry but all he wanted were the right answers and didn't care to learn how to solve them. That simply frustrated me and I gave that up real quick. You have some good helpers here.

Answer 5

i think every one else has answered and hint is that

mol mass of magnesium hydroxide is 58

solve further 2 get the answer

Answer 6

Hey now-------you wont learn that way----the answer is in your book