Formic acid is a weak acid along with a pKa of 3.75. The name comes by the Latin term "formica" ("ant") since this is very plentiful into the secretions of several species of ants, and is exact of the irritants delivered by several stinging ants, wasps and bees.
Assume to make the buffer by adding either strong acid (6.0M HCl) or strong base (5.0 M NaOH) to a solution of concentrated formic acid sequentially to get to the accurate pH. As know about to buy formic acid from Sigma Chemical Corp. as an 88 percent solution (it works out to around 19 M). Please tell me
a. Determine volume of 19 M formic acid to start with
b. Where add HCl or NaOH to acquire to the target pH
c. How many millilitres of strong acid or strong base you suppose need to add (suppose here which the conjugate acid and base concentrations are every equal to 25.0 mM)
Ans:
a. Determine how much stock formic acid required?
2.0 L × (50 mmol/L) = 100 mmol total formate required
100 mmol = 0.100 mol
0.100 mol × (1 L stock/19 mol) = 0.00526 L stock acid needed
= 5.3 mL stock formic acid
b. HCl or NaOH?
Target pH includes getting a mixture of formic acid and formate anion. Therefore, the starting formic acid should be partially deprotonated. It needs use of a base - NaOH.
c. Determine volume of NaOH required?
Hform ↔ H++ form
Start: 100 mmol/2 L
Target: 36 mmol/2 L 64 mmol/2 L
Therefore, should add 64 mmol of NaOH to change 64 mmol of H form to form-.
64 mmol × (1 L/5000 mmol) = 0.0128 L = 13 mL of 5M NaOH