# Help please with this precipitation exercise

• BeeG
In summary: The problem wants you to find the final pH at which the solution will be stable (that is, precipitate will not form). Otherwise there is no way to find the answer without making assumptions about the strength of the acid present.
BeeG
Knowing Ks=6⋅10−38 for Fe(OH)3 in neutral solutions, calculate the minimum pH of an acidic solution in order to completely dissolve 10 mg of Fe(OH)3. Data: Volume = 0.1 L.​

I set out the equation for the equilibrium constant. having first calculated the concentration or iron ions (9.35899*10-4 M), and left the hydroxide concentration as an unknown:
Ks = 9.35899*10-4 M * 27 C3
Solving for C, I get 1.334*10-12 M, and so my guess is that the concentration of the hydronium ions must be the difference, as it has to neutralise the hydroxide ions in order to move the equilibrium towards the left. Doing this, I get 9.35899*10-4 M concentration for hydronium ions, which corresponds to pH = 3.03.
Is this approach and solution correct?

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BeeG said:
my guess is that the concentration of the hydronium ions must be the difference

Difference between what and what?

Do you know what the water autoionization is? You have calculated concentration of OH-. What is pH of this solution?

Borek said:
Difference between what and what?

Do you know what the water autoionization is? You have calculated concentration of OH-. What is pH of this solution?

I do know what water autoionization is. I meant the difference between the initial hydroxide concentration, 9.35899*10-4 M, and the needed concentration for Q = Ks, which is what I calculated as 'C'.

What the problem wants me to do, I think, is calculate the concentration of hydronium ions in an acid solution that I have to add to the one I already have in order to dissolve the precipitate.

Please help. I am at complete loss at what to do and cannot seem to find any guidance in my textbook.

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Is this the exact statement of the problem?

Question is a bit ambiguous.

My bet is that you are asked to find the final pH at which the solution will be stable (that is, precipitate will not form). Otherwise there is no way to find the answer without making assumptions about the strength of the acid present. Not to mention the fact pH can be buffered (in which case it doesn't change during the dissolution).

I gave you hints about how to proceed with such a case in my previous post.

Once you have that and assuming solution contains a strong (fully dissociated) acid it is trivial to calculate (just from the stoichiometry of the dissolution) by how much the initial pH should be lower - but as I said above, it requires an assumption that the acid is strong, and the question doesn't justify this assumption.

## 1. What is precipitation?

Precipitation refers to any form of water that falls from the atmosphere to the Earth's surface. This can include rain, snow, sleet, hail, and other forms of moisture.

## 2. How is precipitation measured?

Precipitation is typically measured using rain gauges, which collect and measure the amount of liquid precipitation that falls in a specific location over a period of time. Snowfall can also be measured by melting the snow and measuring the resulting water volume.

## 3. What factors influence precipitation?

The amount and type of precipitation can be influenced by a variety of factors, including temperature, air pressure, wind patterns, and topography. These factors can affect the amount of moisture in the atmosphere and the way it is distributed.

## 4. What is the water cycle and how does it relate to precipitation?

The water cycle is the continuous process by which water moves from the Earth's surface to the atmosphere and back again. Precipitation is an important part of this cycle, as it returns moisture from the atmosphere to the Earth's surface, replenishing bodies of water and maintaining plant and animal life.

## 5. How does precipitation affect the environment?

Precipitation plays a crucial role in shaping the environment. It can affect the growth of plants and crops, contribute to erosion and weathering of landforms, and impact the availability of clean water for drinking and other purposes. Extreme precipitation events, such as floods and droughts, can also have significant impacts on human societies and ecosystems.

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