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         +         H2O        →        ClO-        +       H3O+
 (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:

pH=pKa+logconjugate baseweak acid


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 pKa

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):

Moles ClO- = 1.68 g NaClO ×1 mol NaClO74.44 g NaClO×1 mol ClO-1 mol NaClO

Moles ClO- = 0.0226 mol ClO-


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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 Ka value for HClO is 3.0 x 10-8 . Enter your answer with two decimal places.

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