Chemistry question on concentration

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AKZ1
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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 10.00
dm3
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
mol dm−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 × 10−10 mol3 dm−9
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Ira Acedia
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What have you tried so far and where are you stuck in particular?
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Ira Acedia
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As a hint:


Spoiler:
Show
There being a different temperature and a new concentration given hints at a change in volume. The solution is still saturated, so the same amount of moles are present too.
Last edited by Ira Acedia; 1 month ago
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AKZ1
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(Original post by Ira Acedia)
As a hint:


Spoiler:
Show
There being a different temperature and a new concentration given hints at a change in volume. The solution is still saturated, so the same amount of moles are present too.
Still dont understand
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Ira Acedia
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(Original post by AKZ1)
Still dont understand
It's a simple moles + Kc question.

Calculate the moles of Mg(OH)2 using the mass needed to make a saturated solution. Every mole of that contains 1 mole of Mg2+ and 2 moles of OH-, so you know the moles of these in said mass.

Mg(OH)2 Mr: 24.3 + 34 = 57.4
Mg(OH)2 Moles = Mass/Mr = 0.1166/57.4 = 0.00203

Therefore there are 0.00203 Mg2+ moles and 0.00406 OH- moles in a saturated solution.

From here, you know the concentration of the OH- moles, as it's given in the question, so you can calculate the volume the OH- ions occupy, which is the same volume that the Mg2+ ions occupy. This volume is different to the volume given in the question as changing the temperature can have a knock on impact on volume. This isn't always the case (sometimes pressure changes, very rarely the number of moles are changed such that pressure and volume remain constant), but it is here.

Now you can substitute values into the Kc equation.
Last edited by Ira Acedia; 1 month ago
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