Mr. banners working at the water treatment plant in his learning about coagulant

Question

Mr. banners working at the water treatment plant in his learning about coagulant aluminum hydroxide. He needs to determine how much aluminum ion can be present in water at pH = 6.5 before precipitation will occur. Find the aluminum ion concentration in milligrams per liter. Mr. banners working at the water treatment plant in his learning about coagulant aluminum hydroxide. He needs to determine how much aluminum ion can be present in water at pH = 6.5 before precipitation will occur. Find the aluminum ion concentration in milligrams per liter.

Explanation / Answer

Given that pH = 6.5

so, pOH = 14-6.5 = 7.5

-log[OH-] = 7.5

log[OH-] = -7.5

[OH-] = 10-7.5

[OH-] = 3.162 x 10-8 M

Let S be the solubility of Al(OH)3

Al(OH)3 -----------> Al3+ + 3(OH-)

[Al3+] = S g/L and [OH-] = 3S g/L

But [OH-] = 3.162 x 10-8 M

So, 3S = 3.162 x 10-8

S = 3.162 x 10-8 / 3

S= 1.05 x 10-8.

Hence concentration of [Al3+] = S = 1.05 x 10-8 g/L

= 1.05 x 10-8 x 103 mg/L........(since, 1 g =103 mg)

= 1.05 x 10-5 mg/L

So, Aluminium ion concentration at pH 6.5 will be 1.05 x 10-5 mg/L

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