# A Level Chemistry question

Please can someone help me with this question

Q10.Use the information below to answer this question.
A saturated solution of magnesium hydroxide, Mg(OH)2, contains 0.1166 g of Mg(OH)2 in 10.00dm3
of solution. In this solution the magnesium hydroxide is fully dissociated into ions.
The equilibrium constant expression for the dissolving of magnesium hydroxide is
K = [Mg2+] [OH−]2 . In a saturated solution of Mg(OH)2 at a different temperature, the concentration
of hydroxide ions is 1.0 × 10-3 moldm-3
Which one of the following has the correct value and units for K under these conditions?
A 1.0 × 10-6
mol2 dm−6
B 5.0 × 10-7 mol2 dm−6
C 1.0 × 10-9
mol3 dm−9
D 5.0 × 1010 mol3 dm−9

sry abt bad format it wont let me format it correctly
(edited 1 month ago)
Original post by hellooooi
Please can someone help me with this question

Q10.Use the information below to answer this question.
A saturated solution of magnesium hydroxide, Mg(OH)2, contains 0.1166 g of Mg(OH)2 in 10.00dm3
of solution. In this solution the magnesium hydroxide is fully dissociated into ions.
The equilibrium constant expression for the dissolving of magnesium hydroxide is
K = [Mg2+] [OH−]2 . In a saturated solution of Mg(OH)2 at a different temperature, the concentration
of hydroxide ions is 1.0 × 10-3 moldm-3
Which one of the following has the correct value and units for K under these conditions?
A 1.0 × 10-6
mol2 dm−6
B 5.0 × 10-7 mol2 dm−6
C 1.0 × 10-9
mol3 dm−9
D 5.0 × 1010 mol3 dm−9

sry abt bad format it wont let me format it correctly
What are your ideas?
Original post by charco
What are your ideas?

My idea was to first find the concentration of the Mg2+ ions. I calculated this by doing 0.1166/58.3 to find the moles which was 2x10-3 then i divided by 10 to find the concentration to be 2x10-4 . Then i did (2x10-4)x(1x10-3)2 to give 2x10-10 mol3 dm-9 but that is not correct so im not sure how to get the correct answer
(edited 1 month ago)
Original post by hellooooi
My idea was to first find the concentration of the Mg2+ ions. I calculated this by doing 0.1166/58.3 to find the moles which was 2x10-3 then i divided by 10 to find the concentration to be 2x10-4 . Then i did (2x10-4)x(1x10-3)2 to give 2x10-10 mol3 dm-9 but that is not correct so im not sure how to get the correct answer

You have worked out the concentration of magnesium hydroxide correctly, but the dissociation produces the same molarity of magnesium and double the molarity of hydroxide ions.

Mg(OH)2 ==> Mg2+ + 2OH-

Your hydroxide ion concentration should be 4 x 10-4