AP Chemistry Practice Exam 1 ❮ 337(c) (i) The sodium hydroxide solution will react with not only the hydrochloric acid (reaction 3) but
also the ammonium ion (reaction 1). The methyl red endpoint is below where the ammo-
nium ion begins to react; therefore, it is a better measure of the amount of HCl reacting than
phenolphthalein, which has an endpoint after at least some of the ammonium ion has reacted.
You get 1 point for this answer.
(ii) The percent will be lower. It will be necessary to use more sodium hydroxide solution to reach the
endpoint, making it appear that less ammonia reacted with the HCl. If there were less ammonia,
then the percentage would be lower.
You get 1 point for predicting the percent will be lower and 1 point for the explanation.
(d)- N Cl
H
H
H H+You get 1 point for having an octet on the nitrogen and an octet on the chlorine with no additional
electrons shown.
You get 1 point for showing an ionic structure with no hint of a covalent bond between the ammo-
nium ion and the chloride ion. The brackets are not essential but are present only as an aid to stress
that these are ions with no covalent (electron-sharing).
Total your points. There is a maximum of 10 possible points. Subtract 1 point if any of your answers does
not have the correct number of significant figures.Question 4(a) Analytical balance, buret, pipette, support stand and clamp, beakers or flask
You must have these five items to get 1 point.
(b) 1. It is necessary to standardize the sodium hydroxide solution.
(i) Pipette a known volume of the standardized hydrochloric acid into a beaker or flask (record
the volume).
(ii) Add a small quantity of indicator to the acid.
(iii) Fill the buret with sodium hydroxide solution and take and record the initial volume.
(iv) Carefully add sodium hydroxide solution from the buret until the indicator indicates the
endpoint.
(v) Record the final volume of sodium hydroxide solution in the buret.
(vi) The difference in the initial volume and final volume from the buret gives the quantity of
sodium hydroxide solution added.
(vii) Use the recorded data and the concentration of the standard acid to determine the concentra-
tion of the sodium hydroxide solution.
The above steps are worth 1 point.- It is possible to determine the molar mass of the unknown acid using the standard sodium hydrox-
ide solution.
(i) Weigh and record the mass of a sample of the unknown acid on an analytical balance.
(ii) Dissolve the unknown in a small quantity of DI water.
(iii) Fill the buret with sodium hydroxide solution and record the initial volume.
20-Moore_PE01_p307-340.indd 337 31/05/18 1:58 pm