10 ml of 0.5 M NaOH is added to 100 ml of 0.2 M formic acid , containing no formate anion.. (the pka of formic acid is 3.75) the resulting mixture has a ph close to:
The answer is 3.3 but how do you get this answer?
millimoles NaOH = mL x M = 10 x 0.5 = 5
millimoles formic acid (HCOOH) = 100 x 0.2 = 20
HCOOH forms HCOONa which is sodium formate and that's the formate anion.
..............HCOOH + NaOH ==> HCOONa + H2O
I...............20.............0..................0................0
add...........................5...................................
C...........-5...............-5...................5...............5
E............15...............0..................5.................5
So here you have a buffer containing 15 mmols formic acid (the acid) and 5 mmols sodium formate (the salt or base), so plug into the Henderson-Hasselbalch equation and solve for pH. I get 3.27 which is close to 3.3.
You have to recognize that when you have a weak acid and you add a base, then a salt of the weak acid is formed. IF the acid is in excess then you see you always have a salt and some acid and that's a buffer.
30 mL of 1 M HCl were added to 200 mL of 0.1 M lysine solution having pH = pKa of the side-chain amino group. What is the new pH? pKa values for lysine are 2.18, 8.95 and (R-group) 10.53.
Answer is suppose to be 5.57
To find the pH of the resulting mixture, we need to consider the reaction that occurs when NaOH is added to formic acid:
HCOOH (formic acid) + NaOH (sodium hydroxide) → HCOONa (sodium formate) + H2O
From the given information, it can be inferred that the amount of sodium formate formed is negligible compared to the initial concentrations of formic acid and sodium hydroxide. Therefore, we can assume that the reaction has approached completion, resulting in essentially the complete neutralization of the formic acid.
To find the pH, we need to calculate the moles of formic acid and sodium hydroxide and determine the final concentration of formic acid after neutralization.
First, let's calculate the moles of formic acid initially:
Moles of formic acid = concentration × volume
= 0.2 M × 100 ml
= 0.02 moles
Next, let's calculate the moles of sodium hydroxide added:
Moles of sodium hydroxide = concentration × volume
= 0.5 M × 10 ml
= 0.005 moles
Since formic acid and sodium hydroxide react in a 1:1 ratio, the moles of formic acid neutralized would be equal to the moles of sodium hydroxide added.
The moles of formic acid remaining after neutralization = moles of formic acid initially - moles of formic acid neutralized
= 0.02 moles - 0.005 moles
= 0.015 moles
To calculate the concentration of formic acid after neutralization, we divide the moles of formic acid remaining by the total volume of the solution:
Concentration of formic acid after neutralization = moles of formic acid remaining / (initial volume + volume of NaOH added)
= 0.015 moles / (100 ml + 10 ml)
= 0.015 moles / 110 ml
= 0.136 M
Finally, we convert the concentration to pH using the pKa value:
pH = pKa + log ([HCOOH] / [HCOO-])
= 3.75 + log (0.136 M / 0.136 M)
= 3.75 + log (1)
= 3.75 + 0
= 3.75
Therefore, the pH of the resulting mixture is approximately 3.75.
To determine the pH of the resulting mixture, we need to consider the reaction between NaOH (sodium hydroxide) and formic acid (HCOOH). This reaction can be written as follows:
HCOOH + OH- -> HCOO- + H2O
First, we need to calculate the amount of formic acid and sodium hydroxide that react. Since 10 ml of 0.5 M NaOH (sodium hydroxide) is added, the moles of NaOH can be calculated using the following equation:
moles NaOH = volume (L) x concentration (M)
moles NaOH = (10 ml / 1000 ml/L) x 0.5 M
moles NaOH = 0.005 mol
Next, we need to calculate the amount of formic acid that reacts. Since the molarity of the formic acid solution is 0.2 M and the volume is 100 ml, we can calculate the moles of formic acid using the same equation:
moles formic acid = volume (L) x concentration (M)
moles formic acid = (100 ml / 1000 ml/L) x 0.2 M
moles formic acid = 0.02 mol
Since NaOH and formic acid react in a 1:1 ratio, the amount of formic acid that reacts is equal to the amount of sodium hydroxide used.
Now, we need to calculate the remaining moles of formic acid in the solution. Since there were initially 0.02 mol of formic acid and 0.005 mol of it reacted, we have:
remaining moles of formic acid = initial moles - reacted moles
remaining moles of formic acid = 0.02 mol - 0.005 mol
remaining moles of formic acid = 0.015 mol
Next, we need to convert the remaining moles of formic acid to its concentration. The total volume of the solution after adding NaOH is 110 ml (10 ml NaOH + 100 ml formic acid). Therefore, we can calculate the concentration of the remaining formic acid:
concentration of formic acid = remaining moles / total volume (L)
concentration of formic acid = 0.015 mol / (110 ml / 1000 ml/L)
concentration of formic acid = 0.136 M
Finally, we can calculate the pH of the remaining formic acid. Since formic acid is a weak acid, we can use the Henderson-Hasselbalch equation:
pH = pKa + log (concentration of formate / concentration of formic acid)
The pKa of formic acid is given as 3.75, and the concentration of formic acid is 0.136 M. Since there are no formate anions initially present, the concentration of formate is 0.
pH = 3.75 + log (0 / 0.136)
pH = 3.75 - log (0.136)
pH ≈ 3.3
Therefore, the resulting mixture has a pH close to 3.3.