Ch. 17 mastering assignment Item 10 10 of 13 The precipitation of aluminum hydro

Question

Ch. 17 mastering assignment Item 10 10 of 13 The precipitation of aluminum hydroxide, AI(OH) 1.3 x 10-3), is sometimes used to purify water Below are some potentially useful conversion factors 1 lb 1 gallon- 4 quartts 1 quart0.946 L 453.6g Part A At what pH will precipitation of Al(OH)s begin if 6.80 lb of aluminum sulfate, Al2(SO4)s, is added to 1150 gallons of water (with a negligible change in volume)? Express your answer numerically to two decimal places. View Available Hint(s) pH-3.83 Correct Part B Approximately how many pounds of calcium oxide, CaO, must be added to the water to achieve this pH? Express your answer in pounds using two significant figures. View Available Hints) lb Submit Request Answer Provide Feedback Next >

Explanation / Answer

(B)

Desired pH = 3.83

So,

Desired pOH = 14-3.83 = 10.17

So,

[OH-] = 10-10.17 = 6.76*10-11

CaO reacts with water to produce Ca(OH)2, which has a Ksp value of 6.5*10-6.

Volume of water = 1150 gallons = 4353 L

Assume that we add 'x' moles of CaO which produces 'x' moles of Ca(OH)2.

So, we have:

[OH-] = (2*x)/4353 = 6.76*10-11

Solving we get:

x = 14713.14*10-11 moles of CaO

This means we have to add negligibly small amount of CaO to achieve this pH. This is because Ca(OH)2 is a base and it will always produce a basic pH in aqueous solution. So if we want to get an acidic pH, then we should not add it at all !

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