# 2.5x10-3 moles of solid Mg(OH)2 is added to a 100.0 mL sample of 0.250 M aqueous chlorous acid solution. Ignore the volume change associated with the added solid. Find the pH of the solution?

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Use the information below to answer the following question:

2.5x10-3 moles of solid Mg(OH)2 is added to a 100.0 mL sample of 0.250 M

aqueous chlorous acid solution. Ignore the volume change associated with the

added solid. Find the pH of the solution.

Use the information below to answer the following question:

2.5x10-3 moles of solid Mg(OH)2 is added to a 100.0 mL sample of 0.250 M

aqueous chlorous acid solution. Ignore the volume change associated with the

added solid. Find the pH of the solution.

##### 1 Answer

#### Explanation:

The base is neutralised by the acid:

The number of moles of chlorous acid is given by:

We are told that:

From the stoichiometry of the reaction we can see that the number of moles of

So the number remaining is given by:

We can also say that the number of moles of

We have now created an acid buffer since we have a weak acid in combination with its co - base.

Chlorous acid dissociates:

For which:

The concentrations are those at equilibrium.

To find

Since we are told the volume is 100 ml = 0.1 L then the initial concentrations become:

At this point it is common to assume that, because the dissociation is so small then

However, we can't do this in this case as the size of

If we multiply out the expression we get:

This is a quadratic equation which can be solved for

If we make the assumption that