# How many moles of sodium hypobromite (NaBrO) must be added to a 2.00 L solution of 0.025 M hypobromous acid (HBrO) to form a buffer solution of pH 9.0 (Ka HBrO is 2.5x10^-9)? Assume the volume of solution does not change on addition of NaBrO?

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a) How many moles of sodium hypobromite (NaBrO) must be added to a

2.00 L solution of 0.025 M hypobromous acid (HBrO) to form a buffer solution of pH

9.0 (Ka HBrO is 2.5x10^-9)? Assume the volume of solution does not change on

addition of NaBrO?

b) If you started with a solution of 0.500 M HBrO and made a pH 9.0

buffer, would the buffer capacity of this system be larger, smaller or the same as the

one in part A. Explain in no more than 3 sentences.

a) How many moles of sodium hypobromite (NaBrO) must be added to a

2.00 L solution of 0.025 M hypobromous acid (HBrO) to form a buffer solution of pH

9.0 (Ka HBrO is 2.5x10^-9)? Assume the volume of solution does not change on

addition of NaBrO?

b) If you started with a solution of 0.500 M HBrO and made a pH 9.0

buffer, would the buffer capacity of this system be larger, smaller or the same as the

one in part A. Explain in no more than 3 sentences.

##### 1 Answer

(a)

0.125 mol

(b)

Larger. See below

#### Explanation:

**(a)**

Hypobromous acid is a weak acid and dissociates:

For which:

Please note that these refer to **equilibrium** concentrations and not **initial** concentrations.

The initial number of moles of

Since

Rearranging

At this point I will make the important assumption that, because the value of

The fact that you are asked to assume that the volume change is negligible is actually irrelevant. Since

This means we can write out

**(b)**

From **amount** of added

This is referred to as the **buffer capacity.**

In (b) to maintain a pH of 9 the concentration of