A 280.0 mL buffer solution initially contains 2.0×10−2 M of HCHO2 and 2.0×10−2 M of NaCHO2. For formic acid, Ka=1.8×10−4. In order to adjust the buffer pH to 4.20, should you add NaOH or HCl to the buffer mixture?

In order for one to adjust the pH of a buffer mixture to a pH of 4.20, one should add HCl to the mixture. This is because the preferred pH is at the acidic range and you should be adding an acid to reach this pH. When adding a base, the pH will be farther from the desired one.

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annyksl annyksl · Answer 2 · 2021-10-29T02:53:39+02:00

For the buffer to reach pH 4.20 it will be necessary to add HCL to the solution.

We can arrive at this answer because:

The pH scale goes from number 1 to number 14.
According to this scale, pH 7 states that the substance has a neutral pH, that is, the substance is neither basic nor acidic.
If the pH is less than 7, the substance is acidic and if the pH is greater than 7, the substance is basic.
If it is desired to make a acidic solution, an acidic substance must be added to the solution.
If you want to make a basic solution, it is necessary to add a basic substance to the solution.

As seen in the question above, it is desired to make the buffer an acidic solution, as its pH should be equal to 4.20. In this case, HCL must be added to the buffer as it is an acidic substance.

It is important to remember that acidic substances are those that have H+ in their compositions, while basic substances are those that have OH- in their compositions.

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