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**Problem**: The pH of 0.50 M HClO is 3.91. Calculate the change in pH when 1.68 g of NaClO (FW = 74.44 g/mol) is added to 31 mL of 0.50 M HClO (FW = 52.46 g/mol). Ignore any changes in volume. The Ka value for HClO is 3.0 x 10-8 . Enter your answer with two decimal places.

###### FREE Expert Solution

We are asked to **calculate the pH change of the solution. **

We can see that the solution has **HClO and NaClO** which is the salt for the conjugate base of HClO.

** HClO + H _{2}O → ClO^{-} + H_{3}O^{+}**

**(weak acid) (conjugate base)**

This means we'll have a buffer.

We use the formula below to calculate the pH of a buffer Henderson-Hasselbalch equation:

$\overline{){\mathbf{pH}}{\mathbf{=}}{{\mathbf{pK}}}_{{\mathbf{a}}}{\mathbf{+}}{\mathbf{log}}\left(\frac{\mathbf{conjugate}\mathbf{}\mathbf{base}}{\mathbf{weak}\mathbf{}\mathbf{acid}}\right)}$

**We go through the following steps to solve the problem: **

**Step 1. **Calculate the moles of ClO^{-}

**Step 2. **Calculate the moles HClO

**Step 3. **Calculate the pK_{a}

**Step 4.** Calculate the pH of the solution with NaClO added

**Step 5. **Calculate the pH change

**Step 1.** Calculate the moles of NaClO

We will use the molar mass or the formula weight (FW):

$\mathbf{Moles}\mathbf{}{\mathbf{ClO}}^{\mathbf{-}}\mathbf{}\mathbf{=}\mathbf{}\mathbf{1}\mathbf{.}\mathbf{68}\mathbf{}\overline{)\mathbf{g}\overline{)\mathbf{}\mathbf{NaClO}}}\mathbf{}\mathbf{\times}\frac{\mathbf{1}\mathbf{}\overline{)\mathbf{mol}\mathbf{}\mathbf{NaClO}}}{\mathbf{74}\mathbf{.}\mathbf{44}\mathbf{}\overline{)\mathbf{g}\overline{)\mathbf{}\mathbf{NaClO}}}}\mathbf{\times}\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}{\mathbf{ClO}}^{\mathbf{-}}}{\mathbf{1}\overline{)\mathbf{}\mathbf{mol}\mathbf{}\mathbf{NaClO}}}$

**Moles ClO ^{-} = 0.0226 mol ClO**

^{-}

###### Problem Details

The pH of 0.50 M HClO is 3.91. Calculate the change in pH when 1.68 g of NaClO (FW = 74.44 g/mol) is added to 31 mL of 0.50 M HClO (FW = 52.46 g/mol). Ignore any changes in volume. The K_{a} value for HClO is 3.0 x 10^{-8} . Enter your answer with two decimal places.

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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Buffer concept. You can view video lessons to learn Buffer Or if you need more Buffer practice, you can also practice Buffer practice problems .

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