When 50 mL of 0.10M formic acid, HCHO2, is titrated with 0.10M sodium hydroxide, what is the pH at the equivalence point? (Be sure to take into account the change in volume during the titration.) Select a good indicator for this titration
To find the pH at the equivalence point, we need to determine the number of moles of formic acid and sodium hydroxide and use the stoichiometry of the reaction to calculate the concentration of hydroxide ions.
Step 1: Calculate the number of moles of formic acid (HCHO2):
Moles = Concentration * Volume
Moles of HCHO2 = 0.10 mol/L * 0.050 L
Moles of HCHO2 = 0.005 mol
Step 2: Determine the stoichiometry of the reaction between formic acid and sodium hydroxide:
HCHO2 + NaOH → HCOONa + H2O
From the balanced equation, we can see that one mole of formic acid reacts with one mole of sodium hydroxide to form one mole of sodium formate (HCOONa) and one mole of water.
Step 3: Calculate the moles of sodium hydroxide needed to reach the equivalence point:
Since the stoichiometry is 1:1, the moles of sodium hydroxide needed will be equal to the moles of formic acid, which is 0.005 mol.
Step 4: Calculate the volume of sodium hydroxide required to reach the equivalence point:
Volume = Moles / Concentration
Volume of NaOH = 0.005 mol / 0.10 mol/L
Volume of NaOH = 0.050 L
Step 5: Calculate the total volume of the solution at the equivalence point:
Total volume = Initial volume + Volume of NaOH
Total volume = 0.050 L + 0.050 L
Total volume = 0.100 L
Now, we can calculate the concentration of hydroxide ions at the equivalence point using the stoichiometry of the reaction:
Moles of NaOH = 0.005 mol
Concentration of NaOH = Moles / Volume
Concentration of NaOH = 0.005 mol / 0.100 L
Concentration of NaOH = 0.050 mol/L
Since sodium hydroxide is a strong base and completely dissociates in water, the concentration of hydroxide ions (OH-) is equal to the concentration of sodium hydroxide.
Step 6: Calculate the pOH at the equivalence point:
pOH = -log10[OH-]
pOH = -log10(0.050)
pOH = 1.30
Step 7: Calculate the pH at the equivalence point:
pH + pOH = 14
pH = 14 - 1.30
pH = 12.70
Therefore, the pH at the equivalence point is 12.70.
To select a suitable indicator for this titration, we need to look for an indicator that has a color change in the pH range close to the equivalence point. Phenolphthalein is a commonly used indicator for acid-base titrations as it changes from colorless to pink in the pH range of 8.2 to 10.0, which is suitable for this titration.
To determine the pH at the equivalence point of the titration of formic acid (HCHO2) with sodium hydroxide (NaOH), we first need to calculate the moles of acid and base present at the equivalence point.
1. Calculate the moles of formic acid:
- Volume of formic acid: 50 mL = 0.050 L
- Concentration of formic acid: 0.10 M
- Moles of formic acid = Concentration x Volume = 0.10 M x 0.050 L
2. Since the reaction between formic acid and sodium hydroxide is 1:1, the moles of formic acid will be equal to the moles of sodium hydroxide at the equivalence point.
3. Calculate the volume of sodium hydroxide at the equivalence point:
- Since the concentration of NaOH is 0.10 M, the moles of NaOH will be the same as the moles of formic acid.
- Volume of NaOH = Moles of NaOH / Concentration of NaOH = Moles of HCHO2 / 0.10 M
After calculating the volume of sodium hydroxide at the equivalence point, we can determine the pH at that point. At the equivalence point of the titration, the moles of formic acid and sodium hydroxide will react completely, resulting in a neutral solution.
4. Calculate the total volume of the solution:
- Initial volume of formic acid = 50 mL
- As the reaction proceeds, the volume of NaOH is added, which is equal to the moles of formic acid divided by its concentration.
- Total volume = Initial volume of formic acid + Volume of NaOH
5. Calculate the concentration of the resulting solution:
- Moles of formic acid + Moles of NaOH = Total moles in the solution
- Total moles / Total volume = Concentration of the solution
At the equivalence point, the resulting solution will have a pH of 7 since it is neutral. To determine the suitable indicator for this titration, we need to know the pH range at which the indicator changes color.
Now let's find a good indicator for this titration:
6. Look up the pH range of commonly used indicators.
- Phenolphthalein, for example, changes color within the pH range of about 8.2 to 10.0.
- Methyl orange changes color within the pH range of about 3.1 to 4.4.
From the pH range of the indicators mentioned above, neither Phenolphthalein nor Methyl orange is suitable for this titration because the pH at the equivalence point is expected to be around 7, which falls outside the pH range of both indicators.
In summary, at the equivalence point, the pH of the solution will be 7, indicating neutrality. However, no suitable indicator can be used for this titration because the pH range of the common indicators does not coincide with the expected pH at the equivalence point.
HCOOH + NaOH ==> HCOONa + H2O
So you have 50 mL x 0.1M = 5 millimols HCOOH to start and you add 5 millimols NaOH so at the end you have 5 millimols HCOONa (sodium formate) in 100 mL or (HCOONa) = 5 mmols/100 mL = 0.05 M. The pH at the equivalence point then is determined by the hydrolysis of the formate part of sodium formate salt as follows:
.............HCOO^- + HOH ==> HCOOH + OH^-
I.............0.05M........................0...................0
C................-x...........................x....................x
E..............0.05-x......................x....................x
Kb for HCOO^- = (Kw/Ka for HCOOH) = (x)(x)/(0.05-x)
Solve for x = (OH^-) and convert that to pH.
Post your work if you get stuck.