4) Students were asked to perform a lab to identify an unknown acid. The experiment has several parts. First the students had to make a sodium hydroxide solution with a molarity of approximately 0.2M. Then the students had to perform a titration with potassium hydrogen phthalate to standardize their solution. Finally they had to perform a titration with the unknown acid to find the pKa and the molar mass, then using their data they had to determine which of the weak acids they had used. Part 1) Make a sodium hydroxide solution A student measured 4.03 grams of NaOH in a tared weigh boat. They then dissolved it into approximately 200 mL of deionized water in a beaker. Once it was completely dissolved the student poured the solution into a 500.0 mL volumetric flask. The student rinsed the beaker with deionized water and added the rinse water to the volumetric flask. They repeated with process, adding the rinse water to the volumetric flask again, they mixed this thoroughly. Finally, they filled the volumetric flask with distilled water, mixing and then waited for the solution to reach room temperature before topping off the solution. A) Calculate the theoretical molarity of the NaOH solution. Part 2) Standardize the NaOH solution. NaOH is a hygroscopic solid, which means that it can absorb water from its surroundings, therefore it is important to determine the actual concentration of the sodium hydroxide solution. To do this the sodium hydroxide solution is titrated with potassium hydrogen phthalate, (KHC8H4O4, Molar mass = 204.23g/mol). KHP is a stable monoprotic solid acid. 1.04 grams of potassium hydrogen phthalate was dissolved into 100.0 mL of water, 2 drops of indicator was added. This was then titrated with the sodium hydroxide solution that was made in part 1. The initial reading of the buret was 1.23 mL and at the equivalence point in the titration 28.74 mL was the reading on the base buret. NaOH(aq) + KHC8H4O4 (aq) ⇌ NaKC8H4O4(aq) + H2O(l) OH-(aq) + HC8H4O4-(aq) ⇌ C8H4O42-(aq) + H2O(l) B) How many moles of KHP were used? C) What volume of NaOH was needed? D) What it the molarity of the standardized NaOH solution
4) Students were asked to perform a lab to identify an unknown acid.
The experiment has several parts.
First the students had to make a sodium hydroxide solution with a molarity of approximately 0.2M.
Then the students had to perform a titration with potassium hydrogen phthalate to standardize their solution.
Finally they had to perform a titration with the unknown acid to find the pKa and the molar mass, then using their
data they had to determine which of the weak acids they had used.
Part 1) Make a sodium hydroxide solution
A student measured 4.03 grams of NaOH in a tared weigh boat. They then dissolved it into approximately 200 mL of deionized
water in a beaker. Once it was completely dissolved the student poured the solution into a 500.0 mL volumetric flask. The student rinsed the beaker with deionized water and added the rinse water to the volumetric flask. They repeated with process, adding the rinse water to the volumetric flask again, they mixed this thoroughly. Finally, they filled the volumetric flask with distilled water, mixing and then waited for the solution to reach room temperature before topping off the solution.
A) Calculate the theoretical molarity of the NaOH solution.
Part 2) Standardize the NaOH solution.
NaOH is a hygroscopic solid, which means that it can absorb water from its surroundings, therefore it is important to determine the actual concentration of the sodium hydroxide solution. To do this the sodium hydroxide solution is titrated with potassium hydrogen phthalate, (KHC8H4O4, Molar mass = 204.23g/mol). KHP is a stable monoprotic solid acid. 1.04 grams of potassium hydrogen phthalate was dissolved into 100.0 mL of water, 2 drops of indicator was added. This was then
titrated with the sodium hydroxide solution that was made in part 1. The initial reading of the buret was 1.23 mL and at the equivalence point in the titration 28.74 mL was the reading on the base buret.
NaOH(aq) + KHC8H4O4 (aq) ⇌ NaKC8H4O4(aq) + H2O(l)
OH-(aq) + HC8H4O4-(aq) ⇌ C8H4O42-(aq) + H2O(l)
B) How many moles of KHP were used?
C) What volume of NaOH was needed?
D) What it the molarity of the standardized NaOH solution?
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Part 3) Determine the identity of a unknown solid
The standardized solution of NaOH was then used to determine the identity of
an unknown acid. 0.785 grams of the unknown monoprotic acid was titrated
using 23.55 mL of the sodium hydroxide solution. The graph below was
created.
HA(aq) + OH-
(aq) ⇌ H2O(l) + A-
(aq)
E) How many moles of base were used?
F) How many moles of the unknown acid were used? (Hint: look at the
balanced equation to find the ratio)
G) What is the molar mass of the unknown acid?
H) What is the pKa of the unknown acid?
I) Based on your answers to G and H, what is the most likely identity of the unknown acid.
J) If you hadn’t standardized the NaOH solution
and you used the concentration from part 1,
A) in your calculation, how would the molar
mass of the acid be different?
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