Consider 6.80 grams of sodium formate (HCOONa) dissolved in 1.00 L of 0.120 M formic acid (HCOOH).
What is the minimum amount (moles) of hydrochloric acid (HCl) required to make the pH fall outside of the effective buffer range (i.e. pH = pKa +/- 1)?

I found the pH of the buffer without any HCL ( 3.67) but I'm stuck afterwards

Question

Consider 6.80 grams of sodium formate (HCOONa) dissolved in 1.00 L of 0.120 M formic acid (HCOOH).
What is the minimum amount (moles) of hydrochloric acid (HCl) required to make the pH fall outside of the effective buffer range (i.e. pH = pKa +/- 1)?

I found the pH of the buffer without any HCL ( 3.67) but I'm stuck afterwards

vincent-lyon.fr · Answer

Step 1. Use the new pH to find out the ratio between [HCOO–] and [HCOOH], then find their amounts in the solution.

Step 2. Write down the reaction produced by the added H3O+

Step3. Work out the amount of H3O+ that leads to the new amounts you found in step 1.