For parts of the free-response question that require calculations, clearly show the method used and the steps involved in arriving at your answers. You must show your work to receive credit for your answer. Examples and equations may be included in your answers where appropriate.
HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)
A student was given the task of titrating a 20.mL sample of 0.10MHCl(aq) with 0.10MNaOH(aq). The HCl(aq) was placed in an Erlenmeyer flask. An equation for the reaction that occurs during the titration is given above.
(a) According to the equation for the reaction, if the amount of the reactants is halved, how does this affect the amount of H2O(l) produced in the reaction?
Question 2
(b) The equation above is not written in net ionic form. Write the correct net ionic equation for the reaction.
Question 3
(c) In the equation you wrote in part (b), which species is a proton acceptor?
Question 4
(d) How many moles of HCl are in the 20.mL sample of 0.10MHCl(aq)?
Answers
Explanation:
Question 1 For parts of the free response question that require calculations, clearly show the method used and the steps involved in arriving at your answers. You must show your work to receive credit for your answer. Examples and equations may be included in your answers where appropriate Answer the following questions about weak acids. Use the K, values in the following table to help answer some of the questions Acid K 6.7 x 10-4 HF HC H30 1.8 x 10-5 (a) Using the expression K. = "HA, explain how to determine which solution has the lower pH 0.10 M HF (aq) or 0.10 M HC,H,O(aq). Do not perform any numerical calculations. (b) Which solution has a higher percent ionization of the acid, a 0.10 M solution of HC,H,O,(aq) or a 0.010 M solution of HC H302(aq) ? Justify your answer including the calculation of percent ionization for each solution 50 100 150 200 Volume of NH Added 300 350 (c) A student titrated 50.0 mL of a 0.10 M solution of a certain weak acid with NaOH(aq). The results are given in the graph above. What is the approximate pK, of the acid? (1) What specific value from the graph is needed, in addition to the information in part (c) above, to calculate the molar concentration of the NaOH(aq) ? (1) Identify a pH value between 2.5 and 7.5 at which the concentration of the weak acid being titrated is less than the concentration of its conjugate base. (d) In another experiment, the student titrated 50.0 mL of 0.100 M HC,H,O, with 0.100 M NaOH(aq). Calculate the pH of the solution at the equivalence point.
Answer: It will contain eight black dots and eight open circles.
Explanation:
Given: HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)
To find: how does this affect the amount of H2O(l) produced in the reaction?
Step 1: (a) Effect of dilution
There will be no effect on the volume of NaOH needed.
The amount of HCl will be halved, so the amount of NaOH will be halved.
However, the concentration of NaOH is also halved, so you will need twice the volume.
You will be back to the same volume as before dilution.
Step 2: (b) Net ionic equation
Molecular: HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)
Ionic: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) ⟶ Na⁺(aq) + Cl⁻(aq) + H₂O(l)
Net ionic: H⁺(aq) + OH⁻(aq) ⟶ H₂O(l)
Step 3: (c) Proton acceptor
H⁺ is the proton. OH⁻ accepts the proton and forms water.
Step 4: (d) Moles of HCl
Moles of HCl
Step 5: (e) Equivalence point\
The equivalence point is the point at which the titration curve intersects the pH 7 line.
Step 6: (f) Schematic representation
Assume the box for 0.10 mol·L⁻¹ HCl contains four black dots (H⁺) and four open circles (Cl⁻).
The 0.20 mol·L⁻¹ solution is twice as concentrated.
It will contain eight black dots and eight open circles.
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